Reach Sensitivity Index Mapping of the Amite River Watershed in the

Journal of Coastal Research

SI

54

141–151

West Palm Beach, Florida

Fall 2009

Reach Sensitivity Index Mapping of the Amite River Watershed in the Lake Pontchartrain Basin: A Tool for Watershed Restoration H. Dallon Weathers{, Miles O. Hayes{, and Jacqueline Michel{ Pontchartrain Institute for Environmental Research University of New Orleans New Orleans, LA 70148, U.S.A. [email protected] {

{ Research Planning, Inc. 1121 Park Street, Columbia, SC 29201, U.S.A.

ABSTRACT WEATHERS, D.H.; HAYES, M.O., and MICHEL, J., 2009. Reach sensitivity index mapping of the Amite River watershed in the Lake Pontchartrain basin: a tool for watershed restoration. Journal of Coastal Research, SI(54), 141– 151. West Palm Beach (Florida), ISSN 0749-0208. In recent years, the focus on issues regarding water quality has shifted from a local site-specific view of a problem toward a holistic view where the health and hydrologic functionality of entire watersheds are of concern. One measure of watershed health is fluvial geomorphology. The goal of this project was to map the morphology of the streams of the Amite watershed of the Pontchartrain Basin using a reach sensitivity index (RSI). The classification scale is based on stream geomorphology, hydrology, and ecological characteristics, where rank is given based on morphological complexity and associated wetlands. The RSI classification method used data from 18 field sites along with digital orthophoto quarter quadrangle 1-m resolution infrared aerial imagery, gap analysis program land cover data, and digital elevation models generated from light detection and ranging. In addition to mapping, issues related to watershed impairment and restoration were identified. Key anthropogenic impacts observed include sand and gravel mining, stream manipulation, profuse litter, and overall water quality. The watershed in this study is proximal to urban areas associated with New Orleans and Baton Rouge, Louisiana, so another theme, waterfront development, was also identified as a source of stream impairment. Reach sensitivity index stream classification can serve as a tool for conservation strategies and watershed restoration, where the ultimate goal is to restore natural hydrological and ecological functionality to impaired streams. Maps generated in the RSI project can further aid in regional planning, without which, rapid urban development and continued stream manipulation within the Amite and neighboring stream systems will result in accelerated watershed impairment and water-quality degradation.

ADDITIONAL INDEX WORDS:

Pontchartrain Basin, Amite River, fluvial geomorphology, environmental, water

quality, reach sensitivity index.

INTRODUCTION The geometry and pattern of streams are dictated by flow characteristics; sediment supply and character; as well as riparian vegetation (Montgomery and Buffington, 1998). Taking that these causative characteristics are in dynamic equilibrium with the shape and pattern of the stream (Leopold and Wolman, 1957), we can look at this relationship in a reciprocal manner, where the observed morphology of a stream reach can be an indicator of these qualities. This article documents a morphologic- and habitat-based reach classification for streams of the Amite River and watershed in Louisiana (see Figure 1). Our goal was to classify and map stream reaches at the watershed scale as a tool for regional planners to use in decision making regarding development, conservation, and restoration.

DOI: 10.2112/SI54-017.1.

Existing Classification Techniques Many classification systems exist for stream morphology. Large-scale classification techniques employ geographic information systems (GIS) or software tools that can quickly analyze large watershed areas; however, adequate data may either be at a resolution that lacks significant detail or altogether unavailable. Other, more detailed classification schemes, such as the Rosgen classification system (Rosgen, 1994), are site specific and predicated on intensive field campaigns to provide indices and metrics that ultimately define morphologic properties of a reach such as ‘‘sinuosity’’ or ‘‘entrenchment.’’ Some watershed scale classification systems that exist include that of Hyatt, Waldo, and Beechie (2004); however, this scheme focuses chiefly on forest as a shade provider, stream buffer, and source of large woody debris for fish habitat, specifically salmonids; and Montgomery and Buffington (1993) outline a robust method for classifying streams based on a wide range of morphologic characteristics, but their reach categories suggest a design for use in mountainous areas. The reach sensitivity index (RSI) classi-

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scale on the basis of stream morphology, riparian forest, and the interaction of the forest and the stream. Special attention is placed on identifying impaired stream reaches and their role in the health of the Amite watershed, and the Pontchartrain Basin as a whole. The goals of the RSI mapping project in the Amite watershed are to (1) Identify stream reaches that are similar in geomorphology and riparian forest associations through classification of stream reaches by the RSI scale. (2) Rank stream reaches with respect to their geomorphologic complexity using the RSI scale. This ranking can also be used in oil-spill response. (3) Identify stream reaches that have been impaired and could benefit from restoration efforts.

Figure 1. The Pontchartrain Basin is located in southeast Louisiana. The map shows the regional location of the Amite watershed that is the focus of this study.

fication method was chosen to provide a reach scale description based on morphology of the stream reach as well as riparian vegetation association. Previously, the RSI has been used to classify streams for watersheds in South Carolina (NOAA, 1996a), Mississippi (NOAA, 1996b), and Puerto Rico (NOAA, 2001). However, these mapping projects were similar to the environmental sensitivity index (ESI) mapping used in large rivers, lakes, and marine coastlines, in which the categorization, ranking, and mapping of the morphology and environmental sensitivity is based on spilled oil, how it will interact with the system, and implications for spill response (Michel, Hayes, and Brown, 1978; NOAA, 2002). Similarly, in these earlier applications of the RSI, oil spill contingency planning was the focus, and parameters related to oil-spill response as well as the potential oiling of the river ecosystem were chief considerations. In this sense, the geomorphology of a stream is recognized as a primary factor that influences tortuosity and turbulence in flow, and is a means by which oil can be spread laterally on the water surface and become mixed vertically in the stream. Wetland and riparian forest associations are identified as biologically sensitive areas where oiling can impart negative impacts on habitat and be difficult to clean up. The dominant sediment size in the stream channel and on depositional bars is important with respect to oil penetration into the river sediments. The final parameter used in previous RSI mapping studies factors in access issues and appropriate settings for oil-spill response and recovery procedures. This study takes the RSI classification system and places it in the context of mapping river systems at the stream reach

Through field observations and the detailed remote analysis of stream reach morphology, natural, unaltered stream reaches were classified and mapped in the Amite watershed. Also, morphologies that indicated stream impairment allowed their distribution and RSI rank to be determined. Most of the impairments were related to alterations of the stream channel and include straightening, dredging, or stream diversion, usually for flood control. Other factors contributing to stream impairment in the watershed include abundant sand and gravel mining adjacent to and in the rivers; poor water quality; and profuse litter.

STUDY AREA The Pontchartrain Basin is in southeast Louisiana and is shown in Figure 1. The city of New Orleans, Louisiana, is at the south shore of Lake Pontchartrain, the primary estuary into which the watersheds of the Pontchartrain Basin drain. Lake Maurepas is a similar, but smaller, estuary to the west of Lake Pontchartrain. It is further up the Pontchartrain Basin and connects to Lake Pontchartrain by Pass Manchac. For this study, the two lakes will be treated as a system and referred to as Lake Pontchartrain, or the lake. The drainage area of the Pontchartrain Basin is 12,173 km2 (Penland et al., 2002), with the vast majority of watershed area occurring to the north of the lake in the Florida Parishes region of Louisiana, as well as parts of Mississippi north of the Louisiana–Mississippi state line. There is additional freshwater input from the south as a result of urban drainage; from the west through Bayou Manchac and other small swampy tributaries; and also from the Bonnet Carre´ Spillway. Input from the spillway can be significant, but input from this source occurs infrequently and is regulated by a gate structure operated by the U.S. Army Corps of Engineers (USACE) as a flood control measure on the Mississippi River (USACE, 2009). Diurnal tides with a mean range of 0.11 m (Sikora and Kjerfve, 1985) and wind driven circulation (Kidinger et al., 2002) mix salt water from the Gulf of Mexico through the Rogilets, Chef Menteur Pass, and the Inner Harbor Navigation Canal. The Amite watershed has a distinct catchment area and drains directly into Lake Maurepas of the Lake Pontchartrain system. It is the largest of the watersheds in the Pontchartrain Basin with a land area of approximately 4822 km2. The main

Journal of Coastal Research, Special Issue No. 54, 2009

Reach Sensitivity Index Mapping

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Table 1. The reach sensitivity index (RSI) scale with a brief description of each class. Distribution of RSI class is also shown by length (in meters, m) and then percentage of total length of streams analyzed in the Amite watershed. Detailed class descriptions can be found in the text.

RSI

Reach Description

1A Quiet water pools with low-sensitive banks 2A Straight channel with low-sensitive banks 2B Straightened or dredged channel with lowsensitive banks 3A Sinuous channel with sand bars and lowsensitive banks 4A Sinuous channel with sand and gravel bars and low-sensitive banks 4B Anabranched channel with low sensitive banks 5A Riffles and pools 5B Rapids over bedrock 6A Straight channel with associated bottomland hardwoods 6B Sinuous channel with gravel bars and lowsensitive banks 7A Sinuous channel with associated bottomland hardwoods 7B Split (braided) channel with sand and gravel bars; some riffles 7C Straightened or dredged channel with associated wetlands 8A Sinuous to meandering channel with associated bottomland hardwoods and a few leakage points into swamps 8B Developed shoreline 9A Sinuous channel with abundant leakage points into swamps 9B Sinuous channel with abundant leakage points into marshes 10A Anastomosing channels with abundant leakage points into swamps 10B Anastomosing channels with abundant leakage points into marshes

Length (m)

Percent (%)

252 15,533 93,272

0 1.8 10.9

57,096

6.6

28,022

3.3

5,177 0 0 11,143

0.6 0 0 1.3

0

0

61,242

7.1

2,531

0.3

129,185

15

256,245

29.8

32,994 76,902

3.8 9

0 89,571 0

Figure 2. Riparian forest types have different degrees of river inundation, duration, and frequency. Cypress–tupelo swamps are almost always saturated, bottomland hardwoods are flooded on an annual or semiannual basis, and upland forests rarely see floodwaters. Modified from Mitsch and Gosselink (1986, figure 14-6; after Clark and Benforado 1981).

data, allowing for multiscale analysis of the watershed from its entire extent at a small scale to large scale, up-close, and detailed analysis. This technique provides the most robust classification of stream reaches by the RSI to date; reaches were mapped and classified in a scale that ranks riparian segments (reaches) with respect to their morphologic complexity using the RSI scale (see Table 1). Detailed descriptions of the riparian wetlands, their role in classification, and the RSI classification scale follow, along with descriptions of field data, digital data, and how these data were merged with digital data to carry out RSI classification.

0 10.4 0

stem rivers and their significant tributaries were mapped and classified using a reach sensitivity index (herein called RSI).

METHODS The RSI is a descriptive, mesoscale geomorphic index that represents a method of stream reach classification and can be used to categorize individual reaches throughout an entire watershed. The stream reach is a geomorphic unit of similarity along a stream and can vary in length depending on the along-channel homogeneity or heterogeneity of factors that influence stream morphology. These factors include channel width, flow velocity, discharge, channel slope, channel roughness, sediment load, and particle size (Leopold, Wolman, and Miller, 1964). A dynamic equilibrium exists as a result of the interaction between these factors and the rate of change in any of these factors; thus, the morphology of a stream reach can be relatively stable or quite unstable. Geospatial data covering the Amite watershed represent the landscape through high-resolution digital elevation models and aerial imagery, as well as habitat data. These remotely sensed data were ground-truthed by field descriptions at individual sites throughout the watershed. A GIS environment is used to organize, overlay, and merge these

Riparian Forests In addition to stream reach morphology, the RSI scale incorporates the degree of association of a stream and adjacent vegetation, such as swamps, marshes, and upland or bottomland hardwood forests (see Figure 2). Although not depicted in Figure 2, a marsh shares the same inundation characteristics as a swamp; however, the vegetation is herbaceous rather than woody. Streams and associated forests provide an excellent habitat, together providing food, cover, and water for land animals as well as aquatic insects and fish. For this reason, stream–forest interaction is taken into account as a key component of reach classification. Figure 2 provides an illustration of three key categories that describe the interaction of a forest and associated stream– upland forests, bottomland hardwood forests, and cypress– tupelo swamps. Each supports a unique assemblage of flora and fauna; however, wetland forests have been identified as both a sensitive and highly productive habitat type. In the RSI, the sensitivity of a riparian forest is related to the frequency and degree to which it communicates with the river itself. Upland forests are considered low sensitive because flooding is infrequent and the duration of inundation is ephemeral. The forest itself is not a wetland, and there are rarely isolated wetlands in this setting. In a bottomland hardwood situation, the forest is flooded on an annual basis, and, in most cases, flooding occurs more frequently. As a result of flooding, the productive capacity of a forest is enhanced by an influx of mineral-rich sediment. In turn, organic detritus is delivered to the aquatic system


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whereby it becomes a basic food source in the system. Also, the bottomland hardwood forest is host to more consistently saturated wetlands. Cypress–tupelo swamps are wetland forests that rely even more on inundation by water. Swamps that are present in a bottomland floodplain are often isolated and removed from the flow of the main stem river in abandoned oxbows and flats. In certain scenarios, the cypress–tupelo swamp is a forest through which a river or channel system flows (Mitsch and Gosselink, 2000). These swamps are flooded more than 50% of the time, so the river and forests are in a frequent state of communication. Because of the abundance of water and organic material, these forests are also highly productive environments for fishes, birds, and other animals. Both the bottomland hardwood forest and the cypress–tupelo swamp communicate with a river during flood events and have each evolved to exist in a specific hydrologic regime. Because these forests are highly productive and require a specific hydrologic regime to function properly, they are considered highly sensitive. Also, they are sensitive to any alteration of the hydrology of the associated river.

The Reach Sensitivity Index The RSI scale used for the Amite watershed (Table 1) ranks stream reaches based on morphologic complexity and wetland associations. A detailed description of each reach class follows. 1A. Quiet water pools with low-sensitive banks. The lowest morphological rank on the scale describes slow moving or ponded water that is flanked with low-sensitive banks (i.e., wetlands generally not present). In this study, there are few ponded reaches, and 1A is used to classify the stream segments that maintain connectivity across larger bodies of water, such as Lake Maurepas. 2A. Straight channel with low-sensitive banks. These are among the most morphologically simple stream reach types and are described as a straight to sinuous channel with lowsensitive banks. The streams are usually smaller headwater streams that flow along a relatively steep vertical gradient such that the channels are fairly straight and flooding is ephemeral and uncommon. Because of this setting, these streams are generally erosional. 2B. Straightened or dredged channel with low-sensitive banks. These reaches are generally the result of creating straight channels to drain or reclaim wetlands for human use, such as agriculture or residence, or to reduce flooding. This is a practice with an outcome that has debatable results. This reach class has much in common with 7C class (see later) with respect to its altered state. Although the preexisting stream morphology and wetland forest associations may have been quite similar between the two stream types in some situations, a 2B reach no longer has associated wetlands and usually represents a greater level of disturbance from the original stream condition than does a 7C reach. Figure 3 provides an example of a 2B reach that is the result of human alteration to the channel.

3A. Sinuous channel with sand bars and low-sensitive banks. In streams of this category, sand has begun to deposit in the form of lateral bars or point bars, depending on the degree to which sinuosity has developed; however, wide floodplains and associated bottomland hardwood forests are usually not present. In places, the lack of bottomland hardwoods is the result of deforestation or streamside development. 4A. Sinuous channel with sand and gravel bars and lowsensitive banks. The morphological character of these stream reaches is similar to 3A reach; however, the bars are composed of mixed sand and gravel. While some uncertainty exists as to the origin of stream gravel in these rivers, it is thought to be a result of river intersection with the gravel rich Citronelle Formation (Self, 1986). 4B. Anabranched channel with low-sensitive banks. While multichannel streams are generally associated with braided, gravel-dominated systems (see 6A) or anastomosed swamp streams (see 10A), a 4B reach is an anabranched channel with low-sensitive banks. This situation could result from an incision in an uplifted swamp (as observed in the Leaf River, Mississippi; NOAA, 1996b) or where an anastomosed swamp has recently been logged. 5A. Riffles and pools. Streams with alternating riffles and pools characterize class 5A reaches. These streams occur in moderate gradients and often contain large cobbles and boulders in which swift moving water flows over shallow riffles and into deeper pools where the water flow is slower. Class 5A stream reaches were not observed in the Amite watershed; it is in the RSI scale as a reference stream reach type that is familiar to many and is widely observed throughout the world. 5B. Rapids over bedrock. The 5B reach (rapids over bedrock) occurs in a steeper gradient where streams are confined to narrow mountain valleys. The morphology that results from this terrain contains turbulent rapids and water that cascades over ledges and into pools. The 5B reach type was not observed in the Amite watershed, but, like the 5A, is present as a common stream type. 6A. Straight channel with associated bottomland hardwoods. A naturally occurring straight stream reach can be associated with bottomland hardwood forests; however, this reach class is often an indicator of channel manipulation. A sinuous or meandering channel is sometimes straightened for flood control or other human need such as a highway bridge crossing. 6B. Sinuous channel with gravel bars and low-sensitive banks. Bars in a 6B reach are pure gravel. Higher ranking is based on the importance of a clean gravel riverbed in which an influx of finer-grained sediment can silt over the gravel and deteriorate the habitat of the reach. 7A. Sinuous channel with associated bottomland hardwoods. This reach is defined by a sinuous to meandering stream channel that floods at a frequency of the annual scale or



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Figure 3. A 2B reach rendered at a scale of 1 : 20,000 in the four major ways that were used for classification. The stream is represented by a white line with interior dotted line. (a) The aerial DOQQ, (b) the DEM, and (c) the GAP habitat data. The habitat data have been simplified for black and white. Open water is white, upland forest light gray, mixed hardwood forest is medium gray, and cypress–tupelo is black. (d) Profile generated from the DEM along the Profile A-A9. Note the signatures of dredging in the DEM that set this manipulated reach apart from a natural straight reach. Light colored spoil banks of locally higher elevation are next to the channel, and the dark crescent shaped features are abandoned channels on the old floodplain. The crescent shapes of the abandoned channels are similar in map view to a naturally abandoned oxbow on the floodplain.

greater. Frequent flooding results in the deposition of alluvial silts onto a floodplain where floodwaters reside for an extended period. Ecologically rich bottomland forests have evolved to survive and thrive in this hydrologic regime. 7B. Split (braided) channel with sand and gravel bars; some riffles. In a 7B reach, gravel bars dominate a braided channel system at average or low flows. During high discharge, the bars are covered by flowing water and may be shifted or relocated because of sediment transport. This bimodal discharge is often seasonal and generally related to events such as spring melt of glaciers or snow pack; however, in this watershed, it is related to the geology of the watershed. This river morphology was rarely observed in the study area, but it is related to sand and gravel mining, which is covered later.

7C. Straightened or dredged channel with associated wetlands. The RSI 7C reach provides an example of an impaired stream in which a meandering or anastomosed channel has either been deepened by dredging or dredged and straightened to simplify the morphology, often in an attempt to provide flood control. As a result, the hydrology has been altered such that the associated wetlands are either drained, or less frequently inundated as compared with how they functioned historically. Other effects of channel manipulation include increased water velocities, bank destabilization, erosion, and, as a result, turbidity. 8A. Sinuous to meandering channel with associated bottomland hardwoods and a few leakage points into swamps. In an 8A reach, the stream is sinuous to meandering with a floodplain that is inundated on an annual basis and, in most years, more


146


frequently. Abandoned meander bends are either connected to the main channel as backwater sloughs or even completely cutoff from the channel as oxbow lakes. These are subject to communication with the main river flow during floods. Cypress–tupelo swamps occupy the sloughs and oxbows as well as other low-lying back swamps on the floodplain. 8B. Developed shoreline. An 8B stream reach is defined by the dominance of human structures such as docks, wharves, or riverside industrial sites. This is an impairment due to the human intrusion on the river directly along with the possibility of nonpoint source pollution and also a long-term presence.

Digital Data Digital data used in this study were obtained from several publicly available, government sponsored GIS data clearinghouses. The primary data used were the digitized stream networks from the National Hydrography Data Set (NHDS), LIDAR-derived digital elevation models (DEMs), GAP Analysis Program (GAP) landcover data, and digital orthophoto quarter quadrangles (DOQQ) color infrared aerial imagery. Following is a detailed description of each of the data sets and how each was used in the mapping and classification process.

National Hydrography Data Set 9A. Sinuous channel with abundant leakage points into swamps. In a 9A reach, the river and associated forest interact to a significantly greater degree than in the 8A class. River reaches of this type have low and subtle banks that allow river flow to communicate freely with the associated wetland forest, which is usually a cypress–tupelo swamp. 9A. Sinuous channel with abundant leakage points into marshes. Like the 9A class, the banks of these streams are low and the stream interacts freely with the wetlands; however, in this case, the riparian system is in a marsh or mangrove environment. 10A. Anastomosing channels with abundant leakage points into swamps. The most morphologically complex of the RSI indices is the 10A, as indicated by Figure 4, where individual channels within a multichannel system are generally highly sinuous and flow through wetland forests that are usually completely inundated. 10B. Anastomosing channels with abundant leakage points into marshes. The stream wetland association in the 10B reach class is the same as in the 10A. However, 10B stream reaches flow through and interact with marsh platforms whose vegetation is herbaceous or, in some cases, mangrove.

Digital stream data served as the geodatabase into which RSI classifications were added. Medium resolution (1 : 100,000 scale) stream network data were retrieved from the U.S. Geological Survey (USGS) National Hydrography Dataset. The USGS and U.S. Environmental Protection Agency published the data in 1999. Streams for the Amite watershed were extracted from the stream network based on the hydrologic unit code (HUC) for the watershed. The data were then projected to the North American Datum of 1983 (NAD83) in zone 15 of the Universal Transverse Mercator coordinate system.

Digital Elevation Models The Louisiana State University (LSU) GIS portal ATLAS is host to the other digital geospatial data. Light detection and ranging (LIDAR) technology allows for large-scale elevation mapping at high ground point density while maintaining detailed vertical resolution. The LIDAR data used in this study were collected in 1999, processed to filter out the vegetative cover, and later published by the USACE, St. Louis District. The data were used to generate DEM grids representing a 5 by 5 m square on the ground at a reported vertical resolution of 0.003 m.

Aerial Imagery Field Data After initial classification of stream reaches using remotely sensed data in a GIS, a field campaign was undertaken from April to June of 2006 where 18 river sites were visited in the Amite watershed. These Amite sites are a subset of a larger database comprising 105 field sites from the entire Pontchartrain Basin. The neighboring watersheds in the basin are located in a similar geologic and climatic setting, so the entire database provides a reference for ground-truthing GIS-based classification with site observed conditions. Boat ramps and bridge crossings were a primary location for field visits because of ease of accessibility; however, at all sites an attempt was made to observe the stream away from physical structures. At each site, the field team carried out a detailed observation of stream morphology in the immediate area. Topographic maps at a 1 : 12,000 scale were available to place the site in context of areas directly up and downstream. In addition to photographs, data forms were filled out, including a field sketch and a field survey form.

Digital Orthophoto Quarter Quadrangles (DOQQ) aerial imagery from 2004 is also available through the LSU ATLAS portal and represents a geographic extent of one-quarter of a 7.5 minute USGS topographic map. The imagery is rendered color infrared, so that variations in vegetation are discernable. The image resolution is 1 m2 on the ground.

Habitat Data Land cover data were used from the GAP, which is the an initiative undertaken by the USGS Biological Division and local agencies to assess and map land cover types and vertebrate species range to facilitate land management. Landsat 5 Thematic Mapper imagery was used in an automated analysis routine to classify sections of land into one of 23 land cover types, including different wetland forest types. A more detailed description of the GAP project, data, and methods by which it was generated is available through the USGS (USGS, 2000).



147

Figure 4. A 10A reach at a scale of 1 : 12,000 by the three digital data sets. (a) Aerial DOQQ imagery, (b) the DEM, and (c) the GAP land cover data, which were simplified for grayscale rendering. Darker gray and black indicate bottomland forest and cypress–tupelo swamps, respectively, while the lighter gray is upland forest. Water is white. (d) Generated from the DEM in (b) along the line A-A9. In the profile, one can see several channels along the floodplain, as well as the walls of the alluvial valley. These channels are not as discernable from the imagery alone, but with the aid of the DEM in map view and the profile, their presence can be discerned. The white triangle with a black outline above and to the right of the center of each frame is the site of a field visit.

Stream Classification Out of the entire Amite stream network, only a subset of the streams of the Amite watershed was taken for analysis. Only streams of order 3 or greater were included in RSI classification. Stream order was determined for streams in the NHD data set following the method of Strahler (1957). Order was assigned to streams from the headwaters of the watershed, all the way to base level at the lake. Because of the scope of the study, only streams in the Louisiana portion of the Amite watershed were included in mapping and analysis. (There is a portion of the watershed that lies in Mississippi, as indicated by Figure 1.) By an iterative method GAP data, DOQQ imagery, and LIDAR DEMs were cross referenced in context of the RSI scale, allowing for confident determination of geomorphology type, presence and degree of association or riparian forests,

and further to identify the different forest types (i.e., upland, bottomland forests, and cypress–tupelo swamps). Aerial imagery provided a first order indicator for stream morphology such as sinuosity, presence of bars, and connectivity with other stream channels. However, processed DEMs were able to highlight ground morphology that was either too subtle or hidden by vegetation to be perceived in the aerial imagery. As such, DEMs served as a secondary source of imagery. The primary use of DEMs was in a quantitative manner by which longitudinal elevation profiles of stream reaches were generated, along with cross-sectional profiles across stream valleys. Smaller profiles at observed breaks in the stream bank were generated to assess connectivity between the stream channels and adjacent wetlands. GAP land cover data were used as the primary source for habitat type. However, because of (1) availability of higher


148


Figure 6. The RSI class distribution for the Amite watershed shows two modes, representing headwater streams (2A and 3A) and main stem reaches (8A and 9A). There is also a significant number of impaired or modified reaches (2B, 7C, and 8C), shown with a wider line.

RESULTS RSI Map and Class Distribution

Figure 5. The map shows the stream reaches colored by the RSI class designation listed in the legend. Also note the black triangles representing the field sites that were visited. In the background, the gray shading behind the Amite watershed indicates Louisiana, while white areas are in Mississippi.

resolution data (i.e., DOQQ imagery and DEMs), (2) incorrect land classification resulting from the coarse resolution of the data, and (3) changes in land cover realized after the GAP baseline data were captured (in 1992), this data set was not solely relied upon for habitat classification. Digital orthophoto quarter quadrangles were also used to determine forest habitat because color infrared rendering of the landscape aids in determining forest type. Evergreen trees often appear red, while darker shades of green were inferred to be wetland forests. Color associations between inferred vegetation types and their representation in aerial imagery were confirmed through comparison with GAP land cover data and descriptions from selected field sites. Also, the DOQQs were obtained 12 years after the GAP analysis and were able to provide a more up-to-date rendering of the landscape and habitat. Figures 3 and 4 illustrate how the three digital data sets were used side by side along with an elevation profile. Annotations point out specific morphological features. It should be noted that cleared wetlands were not recognized, except in cases where other morphologic alterations were observed, such as channel straightening and ditching. Nonforested wetlands were recognized where marshes were present. Results of remote classification were verified by field visits.

The Amite watershed is shown in Figure 5. For this map the stream reaches are color coded according to their RSI class. Thicker lines highlight impaired reach classes 2B, 7C, and 8B. The large catchment area of the watershed results in a stream network of high order; the stream order is 6 by the time it flows into Lake Maurepas. After mapping and assigning RSI classes to the major streams of the Amite watershed, the distribution and occurrence of RSI classes was calculated. Table 1 summarizes the length of stream reaches representing each class and also reflects that number as a percentage of total stream length studied. A graphical summary of Table 1 is shown in Figure 6. When assessing the distribution of stream types, it is important to recall that the subset of streams from the Amite watershed study includes only streams of order greater than 3 that occur in Louisiana. Although present, RSI 1A reaches were not included in the analysis and are not present in this or any RSI distribution figure. This is because most of the 1A reaches represent stream connectivity across large bodies of water such as Lake Maurepas. Other occurrences of 1A streams were rare and insignificant compared with the rest of the RSI classes represented. Another class, the 7B class, which represents braided channels with sand and gravel bars, is barely discernable on the RSI frequency distribution in Figure 6. These only occurred in very small quantities relative to the rest of the reach classes. The Amite watershed is seen to have a bimodal distribution in Figure 6. The smaller distribution peak is related to headwater streams, RSI 2A and 3A, that make up about 8.5% of the total stream length in the watershed. The larger wetlandassociated 7A, 8A, 9(A and B), and 10(A and B) RSI class streams account for the larger mode, accounting for 56.3% of the stream length. One class group that occurs in the Amite, but not in all watersheds of the Pontchartrain Basin, are the gravel associated RSI classes (4A and 7B). These reaches only



make up 3.6% of the stream length in the watershed; however, their occurrence has ties to aggregate mining, and in turn risk of impairment is discussed in the following section. As one of the goals of this study, we identified impaired reaches in classes 2B, 7C, and also 8B that make up 29.7% of the total steam length. In an unaltered state, these impaired reaches would add to the relative abundance of the other reach classes. Reach class distribution, gravelly streams, and reaches from impaired streams classes, along with the causes of impairment, are discussed in greater detail.

DISCUSSION Through classifying stream reaches in the Amite watershed, the occurrence and distribution of reach types were identified, mapped, and quantified, particularly those reaches that are morphologically complex and associated with wetland forests and marshes. Analysis also identified impaired stream reaches through observations that indicated stream manipulation and significant anomalies in stream morphology that set them apart from upstream and downstream reaches in a similar setting. The discussion that follows addresses the RSI classification as it relates to geomorphology in the watershed as well as its use as an indicator of stream impairment, which falls under one of these categories: stream manipulation, sand and gravel mining, water quality, and litter. Some of these impairments, such as water quality and litter, were not observed in remote analysis but were able to be identified through field visits.

RSI Classes and Geomorphology of the Amite Watershed The change in character of streams becomes increasingly complex morphologically and sensitive with wetland associations as the Amite network merges and flows down the watershed. In the Amite River alluvial valley, this is illustrated by the progression of stream types going from the headwaters, through the main stem, and then down to the lake fringe. Lower ranked RSI reach classes such as the 2A and 3A form a secondary reach class mode. These lower RSI classes are most common at the headwater sections of the major streams and are also found in the tributary streams that feed into the larger alluvial valleys. The distribution of classes that represent natural stream channels shows a significant occurrence of reaches that are associated with wetlands, especially classes 7A, 8A, 9(A and B), and 10(A and B). RSI index 7A and 8A streams comprise the general pattern for the major streams that flow through alluvial valleys with floodplains. These floodplains are populated with wetland forests. In the upper reaches of the watershed, these are primarily bottomland hardwood forests with isolated cypress– tupelo forests populating oxbow lakes and some more extensive back swamps. The floodplain environment is a habitat in which there is great biodiversity and biological productivity. The hydrological importance of abandoned channels (oxbow lakes) and back swamps is their role as a water buffer. They hold water through dry periods and

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provide a temporary sink for rising waters during high-flow flood events. As these streams converge and approach the base level of the lake, the channels flow through larger fringing cypress swamps and marshes of water bodies such as Lake Pontchartrain. Here, 9(A and B) and 10(A and B) streams become common. Some interesting reach types that were not overly abundant were those associated with gravels, classes 4A and 7B. This is probably due to the Amite River’s location and size. It is a river system that starts far enough to the north and is large enough to have an alluvial valley that encounters the Citronelle Formation. The Citronelle is a gravel-rich deposit of Plio-Pleistocene age that occurs throughout the Gulf Coastal Plain (Self, 1986). Gravel occurrence is not just of geological interest, but it also relates to stream impairment resulting from aggregate mining.

Impairments of the Amite Watershed As noted previously, the field work and classification activities carried out during this study provided an opportunity to document several sources of stream impairment within the Amite watershed. Four of these, stream manipulation, sand and gravel mining, litter, and water quality impairment, are discussed in the following sections.

Stream Manipulation The most evident stream impairments were anthropogenic manipulation of watershed geomorphology as shown in the 2B (see Figure 3), 7C, and to an extent 6A classes. Alteration to a stream’s configuration and geometry, in turn, alters the hydrologic functioning of the reach. Many of these alterations are projects built with the intent of flood control. However, straightening a sinuous stream locally steepens it, allowing for enhanced stream velocities that can entrain more sediment, causing local erosion and increasing turbidity in the water. Downstream reaches can subsequently become clogged when this eroded sediment is deposited. In addition to effects on hydrology and geomorphology, reach straightening can impact adjacent wetland forests that developed under specific flooding regimes. Reduced flooding also disrupts the symbiotic exchange of mineral nutrients and organic detritus between the stream and the forest by making it less efficient, or even absent. Maps showing impaired stream reaches can be of use for regional planners to identify and prioritize potential areas to carry out stream restoration activities. Other manipulations to the riparian system were related to waterfront development, often in the form of dredged marina networks that occur in the sensitive swamps and marshes, 9(A and B) and 10(A and B), fringing the lake. These types of developments may alter the salinity and habitat of waterways near the lake fringe. Also, these developments artificially increase the water frontage susceptible to household runoff, which can include elevated nutrient levels, as well as a host of other household chemicals. Identification and geographical registering of highly developed waterfronts can aid environmental agencies to target areas for focused water quality monitoring and also quickly identify potential source areas


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for negative inputs to the system when water quality issues are identified.

Sand and Gravel Mining In the Amite watershed, the sand and gravel deposits are being exploited as an aggregate mineral resource. These activities impart both direct and indirect impacts to the river and watershed. Mining aggregate from the floodplain creates lakes. While these abandoned pits are similar in many ways to naturally occurring oxbow lakes, another effect of these features is lowering of the water table, which could in turn change surrounding habitat by dewatering normally saturated soils that would normally support wetland plants. Mining operations are often located in close proximity to the stream, and in some cases, aggregate has been taken directly from the channel. Activities that encroach on stream channels can impart physical change in the rivers (Kondolf, 1997). Head cuts occur as the river gradient is locally steepened by the void left from aggregate removal; as a result, the water velocity is increased and is able to transport more sediment. Simultaneously, the stream is robbed of its bed load. To accommodate this loss, the flow entrains more sediment, thus down-cutting the channel. These effects are not localized because head cuts propagate up the stream network and may have a wide reach. Over time, the stream can become entrenched, and as a result, the floodplains are decoupled from their associated rivers. Once entrenched, high-flow events remain contained inside the channel, where flow velocities are higher and able to entrain more sediment. This leads to local sediment generation that is often associated with increased turbidity and sediment clogging in the channel downstream, as well as bank instability. Aggregate mining can have the same net effect as channel modification for flood control such as increased flow velocity and flow confinement. (This is important to note because stream modification is prevalent throughout the Pontchartrain Basin.) Negative impacts associated with entrenchment and headcutting are enhanced flooding, lowering of the water table, local sediment generation, increased turbidity, and even land loss because the confined stream undercuts the unstable banks to reestablish a stable hydraulic geometry. Also, where agricultural and urban runoff is significant, swifter stream velocities tend to deliver runoff and associated pollutants more swiftly and at higher concentrations to the receiving sink. In this case Lake Pontchartrain is the basin, and a host of issues relating to poor water quality have been identified in these water bodies (LPBF, 2006). However, once these altered and impaired reaches are identified, opportunities do exist for stream restoration through restructuring of the channel to a stable geometry. Also, modification of open and abandoned sand and gravel pits can generate productive aquatic habitat (Norman, 1998).

Litter A large amount of litter was observed in the Amite watershed. The composition and style of litter ranged from

collections of cans and paper trash in treetops or back eddies to appliances, animal carcasses, and bagged trash dumped directly into stream beds at bridge crossings. This problem is at first one of aesthetics; litter is unsightly. However, litter can be of greater ecological harm and cause for deterioration of water quality. In and near streams, deer carcasses and other animal remains can be the source of harmful bacteria; appliances are potential sources of heavy metal contaminants; and the nature of household refuse and bagged trash is also such that it may be the source of toxins from batteries to household chemicals. Public education and availability of suitable means of waste disposal could prevent a portion of this type of water quality and stream impairment.

Water Quality Impaired water quality is well documented and has been an issue of specific concern to groups such as the Lake Pontchartrain Foundation (Penland, Baell, and Mayagarden, 2001). At times, the water quality of Lake Pontchartrain and its tributaries is poor, often due to problems such as fecal coliform bacteria and nutrient loading. These can be persistent problems to the productivity and overall ecological health of the lake ecosystem. At times, levels of contaminants make the water in the lake unsuitable for human contact. Sources of these contaminants start up in the drainage networks that feed into Lake Pontchartrain. Runoff from livestock farms, improper discharge of sewage, and the collective contributions of effluent from high density waterfront development all contribute to poor water quality, particularly fecal coliform bacteria. Improvements in water quality are being realized through rigorous monitoring of water quality, environmental law enforcement, and education of the public about the watershed regarding the importance of water quality to them and their community. However, as long as there are elevated levels of nutrients in streams feeding the lakes of the Pontchartrain Basin, the need for restoration of streams to a natural and hydrologically functional state is clear. Restoration that allows stream flows to be slowed, flooding within the floodplain to occur, and riparian wetlands to be inundated giving natural processes time and contact with contaminant-laden water would allow nutrient levels to be reduced by the time the streams discharge into the lake.

CONCLUSIONS For this study, the ESI mapping concept was adapted and taken up from the Lake Pontchartrain–Maurepas Basin into the Amite River watershed using the RSI mapping method. This article outlines the RSI that was employed to classify and map streams in the Amite River watershed. A data set was generated that shows the occurrence and distribution of stream reach morphologies. This information can serve as a tool for regional planning that is related to identifying areas for conservation, regulating development, and defining restoration priorities. Identification of diverse stream impairments highlights the need for diverse restoration strategies, and also documents the locations where restoration activities are a priority. Physical manipulation of streams and adjacent



floodplains lends itself to restoration activities where the goal is to reinstate a preexisting morphology, or in some cases, create a morphology that enhances the environment beyond its current state of impairment. Priorities for future work in this region include acquiring more detailed data at the reach scale. This will involve use of the RSI data set to identify specific stream reaches where detailed reach scale surveys would be conducted using the Rosgen system, or other similar techniques. Further study can provide detailed and quantifiable information on specific impaired and natural reaches, which can then be used to restore impaired streams in the basin, and in turn, improve the overall water quality and habitat in the basin.

ACKNOWLEDGMENTS This study was conducted as part of the Pontchartrain Restoration Program under NOAA Grant NA04N0S4630255.

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