Correlations between Fluvial Knickpoints and Recurrent Landslide ...

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Oct 28, 2017 - ABSTRACT. This article summarizes an investigation into the likely role of landsliding in the formation of knickpoints along the Indus River in ...
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Correlations between Fluvial Knickpoints and Recurrent Landslide Dams along the Upper Indus River M. FAROOQ AHMED1 Department of Geological Engineering, University of Engineering and Technology, Lahore, Pakistan

J. DAVID ROGERS Department of Geosciences and Geological and Petroleum Engineering, Missouri University of Science & Technology, Rolla, MO 65409, USA; phone: 015733416198, email: [email protected]

ELAMIN H. ISMAIL Department of Geosciences and Geological and Petroleum Engineering, Missouri University of Science & Technology, Rolla, MO 65409, USA; phone: 015732025252, email: [email protected]

Key Terms: Indus River, Landslide Dam, Lithology, Knickpoint, DEM, Longitudinal Profile ABSTRACT This article summarizes an investigation into the likely role of landsliding in the formation of knickpoints along the Indus River in northern Pakistan. The knickpoints and their related geomorphic parameters (channel profile, concavity, drainage area, steepness index, etc.) were extracted from ASTER digital elevation models (DEMs) with 30 m resolution using ArcGIS and Matlab software. In total, 251 knickpoints were extracted from the longitudinal profile of the Indus River along an ∼750km-long reach upstream of Tarbela Dam. The identified knickpoint locations, along with their respective normalized steepness index (ksn values), were compared with the lithologic contacts, mapped faults, a regional-level landslide inventory, and the locations of prehistoric rockslides. The knickpoints identified adjacent to the prehistoric landslide dams (e.g., Katzarah, Gol-Ghone, and Lichar Gah, etc.) exhibited normalized steepness index (ksn ) in the range of 500–1800 m0.9 at various locations along the river channel. The highest normalized ksn values (>1800 m0.9 ) were observed in the tectonically active Nanga Parbat Haramosh Massif region, where the river flows through narrow gorges, and/or where active thrust faults cross the river channel. This study reveals that the landslide dams appear to be one of the significant trigger factors in the formation of knickpoints along the Indus River.

1 Corresponding author email: [email protected]; phone: 0923346435070.

INTRODUCTION Fluvial bedrock incision plays an active role in initiating rockslides that impact channels in mountainous areas (Leopold et al., 1964; Whipple, 2004). The migration of knickpoints through fluvial processes has been studied by a number of researchers, for example, Schumm et al. (1987); Seidl and Dietrich (1994); Whipple (2001); Harbor et al. (2005); Bishop et al. (2005), Crosby et al. (2007); and Ahmed and Rogers (2013), among others. These studies have shown that the upstream migration of knickpoints plays a significant role in initiating bedrock incision along stream channels, even triggering changes in the local base level. The most common mechanisms of concentrating detrital influx (choking the channel) are tributary accretion at the mouths of major tributaries and landslides and/or rockslides. Landslide dams often trigger bedrock incision by shifting the point of down-cutting onto the opposing, un-failed river bank. Most landslide dams tend to constrict channels and divert flow to the distal margins of debris dams, where overtopping often ensues, cutting downward (Ahmed and Rogers, 2014). These accumulations of coarse debris hinder the river’s transporting power and eventually retard the erosion process, if the blockages cannot be dislodged by normal river flow. This situation often occurs at the confluence of tributaries with main channels or at the foot of large landslides (Crosby et al., 2007; Ahmed et al., 2014). The oversize blocks left in the channel usually are found in clusters sufficient to form rapids that retard subsequent channel flow, locally increasing the channel gradient. These features are often observed as knickpoints in the longitudinal profile (Morisawa, 1960; Leopold et al., 1964). These knickpoints typically

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exhibit steep downstream and gentle upstream slopes, and they resist bedrock erosion and upstream incision (Wang et al., 2012). In their study of the eastern margin of the Tibetan Plateau in Sichuan, Ouimet et al. (2007) noted the importance of large landslides in forming stable knickpoints. These knickpoints exert local control on river morphology and the channel profile by inhibiting incision and preventing the adjustment of channel beds to regional tectonic, climatic, and lithological factors. In most instances, this resistance to bed erosion appears to be a result of the coarse debris serving to “armor” the channel in these locations, while leaving a locally disturbed longitudinal profile (Hewitt, 1998; Korup, 2004a). In other words, a “step pool system” (Wang et al., 2012) is often formed by the rearrangement of the oversize bedrock blocks and boulders that have slid into the channel, obstructing and absorbing much of the flow energy, and thereby reducing bedrock incision (Whittaker and Jaeggi, 1982). There are many short-lived rockslide avalanches and glacial/moraine dams concentrated in the high-relief zones of the western Himalayan syntaxes, along the Indus, Gilgit, Hunza, and Shyok Rivers (Hewitt, 1982, 1998, 2002, 2009; Shroder and Bishop, 1998; Korup et al., 2010; Hewitt et al., 2011; and Ahmed and Rogers, 2014). The slope morphologies expressed at most of these mapped landslide dam sites (Ahmed and Rogers, 2012, 2014) suggest that the hillslopes were perturbed by previous episodes of landsliding and are continuously eroding. Each event truncates evidence of previous landslide sequences/events, making the older events increasingly difficult to discern, morphologically. The terraces observed along the Indus River tributaries in the Nanga Parbat Haramosh Massif (NPHM), as well as most of the Karakoram and Himalayas, appear to be composed of remnants of prehistoric landslides. In this study, the knickpoints identified along the Indus River’s longitudinal profile were analyzed, with a focus on measured normalized steepness indices (ksn ), to understand whether these knickpoints might be utilized as indirect criteria to identify the sites of unmapped landslide dams and/or rock avalanches. STUDY AREA This study was carried out along the Upper Indus River, which plays a significant role in the overall drainage systems within Pakistan. The river originates from the Tibetan Plateau, flowing through northern Pakistan from the Kashmir region through the entire country, eventually terminating at the Arabian Sea to the south (Figure 1). The irregular longitudinal profile of the Indus River is caused by tectonically active

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thrust faulting and associated uplift, especially across the NPHM region (Leland et al., 1998; Shehzad et al., 2009; and Korup et al., 2010). In this area, the rates of bedrock incision can be as much as 12 mm/yr. However, in many stretches, the river is obstructed by a significant number of landslide dams with volumes exceeding 10 × 106 m3 , which are expressed in the river’s longitudinal profile as prominent knickpoints. The most likely trigger factors for the high density of mapped slides (other than lithology) could be the high seismicity (Keefer, 1984) associated with a number of active thrust faults and periodic increases in the river discharge volume associated with climate variation actively triggering incision, which results in increased landslide activation. At the locations of historic landslide dams that have breached (e.g., Gol-Ghone, Katzarah, and Lichar Gah, etc.), the river is still actively down-cutting through the slide debris (Hewitt et al., 2011; Ahmed and Rogers, 2014). The enormous volumes of slide debris choking the main channel of the Indus appear to protect the Tibetan Plateau from more rapid dissection (Korup et al., 2010). This situation is also observed in the Skardu Valley (Figure 1), where the river flows upon a thick sequence of alluvial detritus. Most of the valley’s channel terraces have been stabilized by vegetation and support argillic B soil horizons, indicating considerable periods of subaerial exposure, especially, in the center of the river valley.

DATA AND METHODOLOGY Data The regional geomorphic analysis of the Indus River longitudinal profile was performed using data extracted from the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) global digital elevation model (GDEM) v2 data tiles, with 30 m resolution, using ArcGIS 10.2 (http://www.esri. com/software/arcgis/arcgis-for-desktop) and Matlab 12 (https://www.mathworks.com/products/matlabproduction-server/) software (Ahmed et al., 2017). The ASTER GDEM v2 is pre-processed, freely downloadable global elevation data with minimum artifacts, spikes, and errors. This data set provides reasonably accurate elevation measurements over Shuttle Radar Topography Mission (SRTM) 90-m-resolution data in steeply inclined terrain (Tachikawa et al., 2011). The GDEM data tiles were mosaicked and geo-referenced with Pakistan Grid Map World Geodetic System (WGS) 1984 Universal Transverse Mercator (UTM) Zone N430 using Envi 4.8 software (http://www. exelisvis.co.uk/ProductsServices/ENVIProducts.aspx)

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Figure 1. The upper Indus River basin above Tarbela Dam. The study area is outlined in black along the main stem of the upper Indus River in northern Pakistan.

to perform the regional geomorphic analysis in the ArcGIS and Matlab environment.

The ksn values were obtained by modifying the relationship given in Eq. 2 (from Whipple, 2004; Wobus et al., 2006),

Methodology

ksn = ks A(␪ref − ␪) ,

The morphology of the river’s longitudinal profile can be explained by a variety of stream power models (Montgomery, 1994; Whipple, 2004; and Wobus et al., 2006). The basic stream power relationship between different geomorphic parameters is given in the following equation:

where ␪ ref is the reference concavity, and ␪ is the observed concavity of the stream channel at different channel segments. The ␪ ref value can be computed by averaging the local ␪ values observed along the respective channels for multiple reaches. In steady-state river systems that are in equilibrium with tectonic, climatic, or other environmental conditions, the longitudinal stream profile can be modeled by a single combination of steepness index and concavity. However, in transient river systems or river reaches where the stream profile presents abrupt changes in channel gradient, convex reaches may be found. These phenomena are called knickpoints (Wobus et al., 2006). In this analysis, multiple ksn values were generated for short segments of all the rivers in a catchment above a minimum drainage area to identify and characterize individual knickpoints. To obtain representative values of steepness indices, appropriate values of the reference concavity (␪ ref ),

S = ks A − ␪,

(1)

where S is the local slope of the channel; A is the mid-point area (Acent ) of the segment analyzed in the regression analyses in the form of Acent = 10(log Amax + log Amin )/2 (Wobus et al., 2006); ks is the local steepness index, which is the ratio of the channel gradient at specific locations (knickpoints) in the drainage area; and ␪ is the concavity of the stream channel profile (Kirby and Whipple, 2001; Wobus et al., 2006). The values of A and S could be assessed from regression analyses (Montgomery, 1994; Wobus et al., 2006). The value of ksn , expressing channel steepness, is normalized with respect to the upstream drainage area.

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Figure 2. (A) Longitudinal profile of the Indus River upstream of Tarbela Dam, generated by Matlab 2012, using ASTER DEM 30-mresolution data. (B) An enlarged portion of the plot, illustrating the actual and smoothed channel profiles with user-specified knickpoints. Notice the two consecutive major knickpoints having ksn values of 3,225 and 2,139, respectively, observed a few kilometers upstream of Nanga Parbat Haramosh Massif (NPHM) region.

smoothing window size (250 m), and sampling contour interval (10 m) were selected to extract longitudinal profiles from the ASTER DEM data. The ␪ ref value of 0.45 was taken (which is a typical value for rivers in active mountain belts) for the Indus River channel in the Himalayan Region (Whipple, 2004). The relative steepness indices (ksn ) for different river reaches were obtained by applying a linear regression relationship between log slope and log drainage area for a fixed concavity index (␪ ref = 0.45) in the Matlab environment. The term (␪ ref − ␪) was calculated from the channel profile, considering the channel concavity at each point, and then it was used to estimate the normalized steepness index (ksn ) values. The user-identified knickpoints were marked within Matlab on the extracted longitudinal profiles. The user-identified and marked knickpoints with their respective ksn values (Figure 2) were then exported to ArcGIS to compare them with the mapped landslides and related features (Ahmed and Rogers, 2014). Korup et al. (2006) also followed similar criteria to verify whether the ksn values of channel segments were influenced by landslides, by noting if these values were significantly dissimilar from the rest of the channel profile. Higher ksn values, >175–2,000 m0.9 , were observed for those river segments previously impacted by sizable rock slope failures in the Indian Himalayas, Tien Chen (China), and the Southern Alps in New Zealand (Korup, 2004b; Korup et al., 2006). In a recent study (Ahmed et al., 2017), 251 useridentified knickpoints were marked at different places along the Indus River’s longitudinal profile (see Figure 2). The green line shows the actual longitudinal profile, while the cyan color line delineates the smoothed longi-

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tudinal profile using the reference concavity value (␪ ref = 0.45) over an average 250 m reach of channel with 10 m contour interval in Matlab environment. This specification is appropriate to minimize the spikes, noise, and artifacts from the DEM data to highlight notable knickpoints along the channel sections (Ismail and Abdelsalam, 2012). The user-identified knickpoints were divided into “major” and “minor” groups, depending upon the magnitude of the elevation drop (difference in elevation). Using these criteria, there were 134 major and 117 minor knickpoints identified along the Indus River profile. “Major knickpoints” were arbitrarily characterized as those expressing a significant drop in elevation over a relatively short horizontal distance (i.e., >10 m vertical drop over an averaged 250 m reach on the profile). “Minor knickpoints” were those that exhibited topographic features similar to major knickpoints, but with less elevation change (i.e., 15 m vertical drop over an averaged 250 m reach on the profile) were identified where mapped faults crossed the river channel. The enlarged view of the longitudinal profile (Figure 2B) clearly shows marked major and minor knickpoints. This section is taken along the Indus River where it enters the NPHM region. In this area, the river appears to be structurally influenced by linear features, most likely faults. At a horizontal distance of 540 km from Tarbela Dam, the river takes a sudden 90 degree turn, which likely follows another linear feature. The very gentle gradient (nearly horizontal) between these two consecutive knickpoints might be because the river is flowing parallel to these tectonic features, which are lifting it at near-constant rates.

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Figure 3. Plot illustrating the comparison of landslide dams and other geomorphic features with the knickpoint locations along with the observed ksn values on the longitudinal profile of the Indus River upstream of Tarbela Dam. Note the location of MMT fault zone, Stak (2), Baroluma (6), Raikot (8), and other faults (modified from Ahmed et al., 2017).

Figure 3 exhibits the variation of normalized ksn values (hollow circles), adjacent to the user-identified knickpoints along the profile. Lithologic contacts are provided as different color bars, and active faults are indicated as dotted lines. The thin blue lines denote prominent landslide/rock avalanche dam features with their likely ksn values. Landslide Inventory Maps and Prehistoric Landslides Data gleaned from the regional landslide inventory maps of the Indus River corridor (Ahmed and Rogers, 2012, 2014) and documented rockslides within this corridor (Hewitt, 1982, 2002; Shroder and Bishop, 1998; Korup et al., 2010; and Hewitt et al., 2011) were utilized to compare the spatial distribution of knickpoints (along with their associated ksn values) with mapped landslide features. Anomalous topographic protocols, which focused on bedrock slides more than half a kilometer long, were utilized to identify landslide features along the Indus River (Ahmed and Rogers, 2014). The topographic keys included: descending slopes with evidence of isolated topographic benches, divergent contours, crenulated contours, arcuate headscarps, extended topographic ridges, isolated topographic benches, and sudden up/downturns in hillslope contours (Rogers, 1980, 1994; Cruden and Varnes, 1996; Crosta, 2001; Glade, 2001; Doyle and Rogers, 2005; Van Den Eeckhaut et al., 2005; and Ahmed and Rogers, 2014). This regional inventory tentatively identified more than 2,200 deep-seated bedrock slides and secondary flow slides.

The landslide inventory map included 451 landslide features along both sides of the Indus River channel. These identified landslides were used to validate this study. Most of the bedrock landslide features identified were more than 1 km in length along the direction of landslide motion, and they appeared to be structurally controlled by bedding or foliation, pervasive jointing, lithologic contacts, and/or active faults. Subsequent movements, usually not related to the rock discontinuities, can be due to toe erosion, surcharging by adjacent slide debris, and earthquakes. The flanks of the parent bedrock could be fractured and subject to erosion, resulting in secondary slope failures as rotational slumps and earth flows. These secondary slope failures can move into the river channels, displacing the channel or forming significant knickpoints (Ahmed and Rogers, 2014). These secondary movements often trigger smaller slump blocks, flows, and rock avalanches. The mapped landslide features were spatially correlated to many of the documented historic landslide dams and rockslide features along the Indus River (Hewitt, 1982, 2002; Shroder and Bishop, 1998; Korup et al., 2010; and Hewitt et al., 2011). This provided an encouraging indication of validity for a regional-level study (covering a land area of >18,000 km2 ) and also made the mapped landslide features suitable for further utilization in the current study. Skeletal Blocks as Persistent Knickpoints The apparent preservation of landslide dams as knickpoints suggests that coarse debris remaining in the channel likely retards the rate of bedrock incision

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Figure 4. (A) Schematic section view of initial conditions through the inner gorge of an asymmetric bedrock canyon, sculpted by rock joints and foliation. (B) Schematic juxtaposition of blocks in a multiple block-glide rockslide emanating from a bank, along pre-existing suites of discontinuities. The blockage can then serve to elevate the river surface and water table in the adjacent banks, as shown on the left side at the profile. (C) Sketch illustrating the impacts of sudden drawdown of the local water level triggered by a landslide dam outbreak flood. This sudden drawdown often leads to new failures occurring along formerly submerged banks, on the opposite side of the channel.

at these locations. This inadvertent armoring could also impact local base levels as much as several kilometers upstream of old landslide dam sites (as observed at the Gol-Ghone and Katzarah sites). Thousands of years may pass before the river erodes the landslide debris and recovers its equilibrium grade. In most instances, the landslides appear to recur at the same locations, resulting in an accumulation of “skeletal blocks” at that location. This may explain why “major knickpoints” are formed at sites that do not exhibit overt tectonic offset along active fault zones. Figure 4 illustrates the typical mechanisms by which landslide dams tend to recur at the same general locations in the study area, likely because each blockage may influence and/or disturb the local equilibrium. After the initial slope failure occurs along pre-existing sets of discontinuities, it leaves an unsupported slope in the headscarp evacuation scar. The accumulated debris at the toe of the slide is then eroded by the river and subsequent slope failures and partial reactivations may

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ensue, depending on how much debris is dispersed or removed by the river. Subsequent movements may be caused by rapid drawdown of the groundwater in the banks during catastrophic “outbreak floods” that breach the landslide debris dams. Figure 4A shows the initial site conditions along a hypothetical channel reach exhibiting the asymmetry typical of opposing canyon walls, common in layered or foliated rocks (typical of the Himalaya). The morphology of most canyon slopes is influenced by pervasive discontinuity suites, shown in this schematic view. Earthquake shaking or a rapidly falling channel (after debris dam is breached) can trigger mobilization of the joint-bordered blocks, which then obstruct the main channel. If a sufficient volume of debris effectively blocks the channel, the river’s flow can be temporarily suspended until it begins to overflow the debris dam. Upon breach, the flow is usually concentrated to the topographic low side of the dam, which is typically the opposing bank of the channel, as shown by the yellow alluvial gravels in Figure 4B. Such masses of blocky debris can obstruct the channel for considerable periods of time, between a few decades to as much as a few thousand years (Schuster and Costa, 1986). In this case, a temporary reservoir would be impounded upstream of the blockage (Grater, 1945). If the reservoir persists for more than a few weeks, the groundwater table will rise within the submerged banks, as sketched in the left side of Figure 4B. Figure 4C presents a conceptual view of a landslide dam shortly after overtopping. The water stored behind the debris dam will rapidly excavate a new channel, cutting downward from the point of initial overtopping, usually on the opposite channel bank (Rogers, 1994). Additional slide debris can be expected to slide and erode into the channel as the dam is rapidly excavated and debris is moved downstream. The rapid drawdown of the temporarily elevated groundwater within the debris and valley sides associated with the rapid erosion of the debris dam often triggers new block movements along inclined discontinuities at the lower extremities of the opposing slope, as sketched in Figure 4C. Note the possible recurrence of a multiplicity of slides, not only at old landslide dam sites, but along either bank of the temporary reservoirs formed behind debris dams. Also note that the displaced blocks on one bank are often semi-stabilized and in-filled with finer debris. The sequence of events presented in Figure 4A, B, and C suggests that the impact of sizable rock slope failures on a river’s longitudinal profile can be significant and is likely worthy of more attention in fluvial geomorphology, similar to other geomorphic parameters, such as climatic and tectonic factors.

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Figure 5. Landslide inventory map (modified after Ahmed and Rogers, 2014) with red and yellow dots denoting the exported knickpoints (from ArcGIS software). The blue ovals show highlighted areas where the landslides and active faults apparently have significant correlation with identified knickpoints; a few of these areas were selected for further discussion.

Figure 6. Excerpt of landslide hazard map showing the area surrounding the prehistoric 450-m-high Gol-Ghone landslide dam. Outlined red circle shows the probable extent of the landslide dam, whereas the arrows show movement of the rock avalanche from the valley sides along the river: A cross-valley profile upstream of the Gol-Ghone landslide dam (C–D) is shown at lower left, while a cross-valley profile through the middle portion of the landslide dam (A–B) is presented at upper right.

RESULTS AND DISCUSSION Description of a Few Landslide Dams and Persistent Knickpoints in the Himalayas The identified knickpoints with their respective normalized steepness index (ksn ) values were exported to ArcGIS software and co-registered on the hillshade topographic map (Ahmed and Rogers, 2014) with the topographic features interpreted to be river-damming landslides. The highlighted areas, shown on Figure 5 as circles and ovals, were chosen to evaluate the likely association between the observed knickpoints and the landslide dams. The locations of a few mapped larger prehistoric landslide dams (volumes >10 × 106 m3 ) are shown on Figure 5. This figure also reveals that significant knickpoints, or “steps,” occur where active faults cross the channel, or where the channel has been obstructed by the landslide dams. For these larger prehistoric events, such as Gol-Ghone, Katzarah, and historic Lichar Gah, 1841 (see Figure 5), the normalized steepness index (ksn ) increases markedly. Figure 6 presents the prehistoric (Holocene age) GolGhone landslide dam, which impounded a reservoir at least 450 m deep (Hewitt, 2002; Hewitt et al., 2011). This mega-event (>10 km3 ) occurred just downstream of the confluence of the Shyok and Indus Rivers. At this location, the Indus River has incised 400 m through landslide dam debris, with another ∼50 m of debris still lying beneath the current channel. This debris deposit serves as the local base level (Korup et al., 2010; Hewitt et al., 2011). In this area, the Indus River flows through

a narrow gorge, and a number of knickpoints were observed within the footprint of the old landslide debris dam. One of those knickpoints exhibited a ksn value of 877 m0.9 , suggesting a major channel blockage from large resistant blocks in the breached debris dam. Cross-valley profiles (Figure 6) were developed through the center of the old landslide dam (section A–B) and approximately ∼2 km upstream of the dam (section C–D). The section through the landslide barrier is rather narrow and linear, indicative of a channel that is rapidly down-cutting its bed and flowing on a fairly high gradient, with numerous rapids. A few kilometers upstream, the channel is noticeably wider and shallower, due to aggradation along a reach exhibiting a much lower hydraulic gradient. Note the significant topographic benches 300 to 800 m above the present channel formed by the massive slope failures (section A–B). One of the largest documented, more likely prehistoric, rock avalanche features (Hewitt et al., 2011) in the study area is at Katzarah, on the west side of the Skardu-Shigar Basin. Figure 7 presents an example of the geomorphic impacts of catastrophic rockslide avalanches, including shallow braided channels and backwater aggradation upstream of the blockage, resulting in rapid deposition of braid bars, fine sand, and lacustrine silts downstream of the slide. Large remnant blocks from the old debris dam are scattered around the footprint of the breached landslide dam. In this area, the river has yet to retrench itself to the pre-slide riverbed profile. Figure 7 also shows a major

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Figure 7. Landslide inventory map surrounding the Katzarah landslide dam location. Outlined red circle shows the probable extent of the landslide dam, whereas the arrows show movement of rock avalanche from the source along the river. A major knickpoint exists just downstream of the dam site, exhibiting a ksn value of 1,610 m0.9 . An aerial view of the Katzarah rock avalanche debris dam is shown at lower left, which shows the areal extent of the source area and debris fan developed in the Indus River channel, as well as the remnant lake.

knickpoint downstream of this dam site, which exhibits a ksn value of 1,610 m0.9 . This knickpoint might have been developed by some other landslide event, since the area is densely mapped with slides, or from the possible migration of debris downstream of the main event in the past several thousand years. A few kilometers downstream of Katzarah, the Indus River enters a very narrow chasm, with numerous knickpoints (Figure 7). This steep-sided gorge is armored with large boulders and skeletal blocks eroded from other landslide debris that once filled the channel. Further downstream, the Indus River turns sharply, entering the NPHM region (see Figure 8). This area comprises the northwestern part of the Greater Himalayas, south of Kohistan. The whole region is blanketed by mapped landslide features (Ahmed and Rogers, 2014), and significant numbers of knickpoints were identified in this reach. On the eastern flank of the NPHM region, a cluster of major and minor knickpoints was observed, with exceptionally high normalized steepness index (ksn ) values (>1,000 and up to 4,196 m0.9 ). Figure 9 shows what appears to be a breached landslide dam with identified knickpoints a few kilometers upstream of the Main Mantle Thrust (MMT) zone. The knickpoint with the highest ksn value (1,876 m0.9 ) occurs at the site of a former landslide dam (see photo at top right of Figure 9). Along the western flanks of the NPHM, downstream of the Indus-Gilgit River junction, the river becomes a braided channel where it flows through a Quaternary-

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Figure 8. Excerpt of landslide inventory map where the Indus River flows through a steep-sided gorge in the eastern side of the NHPM, near the confluence of the Gilgit River, which flows through a broad, flat valley.

age alluvial valley (Figure 8). Widening of the channel in this reach likely resulted from the significant influx of sediment from both the Gilgit and Hunza Rivers, which have an aggregate watershed of ∼26,000 km2 . This aggradation sequence is locally restricted because of the increased stream power and active uplift associated with the MMT, as well as a complex system of active faults, such as the Raikot, Baroluma, and Stak Faults (Dipietro et al., 2000; Hewitt et al., 2011). Further downstream, in the Lichar Gah area (shown in Figure 10A and B), a seismically triggered rockslide in 1841 blocked the main Indus River channel (Code and Sirhindi, 1986). This dam was breached

Figure 9. Portion of the landslide inventory map a few kilometers upstream of MMT in the NHPM region. The image at upper right shows the breached landslide dam section (photo by Arve Tvedt, 2011).

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Figure 10. (A) (a) Ground view of the current situation, where the channel cuts through the dissected landslide dam, and (b) image showing the severe incision of the channel through sediment deposited a few kilometers upstream of the landslide dam (photo by from A. Mughal, 2010). (B) A portion of the landslide inventory map showing the Lichar Gah landslide dam location (outlined in red), triggered by an earthquake in 1841.

catastrophically after 6 months by significant inflows produced by excessive rainfall and snowmelt the following summer. The knickpoint associated with the breached Lichar Gah landslide dam exhibited one of the highest steepness index values (ksn = 2,951 m0.9 ) of any of the landslides identified along the main channel (see Figure 10B). Figure 11A (a–d) shows the physical characteristics of the Indus River channel at various locations upstream of the mapped Kes Gah landslide dam. These images show remnants of lacustrine silt and clay deposits that were trapped in the reservoir formed by the landslide dam. Figure 11B presents an excerpt of the landslide inventory map of this same area, where

the anomalous topographic features suggest that a series of debris dams likely blocked the channel in this reach. Note the mapped landslide features along with their respective knickpoints in the Kes Gah area. These knickpoints were probably generated from these masswasting events, as there is no evidence of fault activity or lithologic contrasts in this area. Figure 12 shows the current course of river flow near Hodar Gah, a few kilometers downstream of Kes Gah, along the Indus River (photo on lower left). The landslide inventory interpreted this area as a series of breached debris dams, with a buried channel lying beneath the largest run-out fan, causing the present channel to make a hairpin turn around this seemingly recent (but prehistoric) blockage. The hillshade topographic

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Figure 12. Excerpt of the landslide inventory map showing the locations of probable landslide dams, a few kilometers downstream of Kes Gah area along the river. Ground view shows the breached section of the river near Hodar Gah, looking upstream (photo by Der Fuchs, 2007).

Figure 11. (A) Indus River channel near the Kes Gah landslide dam: (a) current incised channel conditions about 6 km upstream, (b) evidence of river deposits on the adjacent slopes before breaching, almost 3 km upstream from the site of the mapped landslide dam, (c) breached section showing the landslide debris close to the Kes Gah event, and (d) a little downstream of the Kes Gah landslide dam. (B) Map showing locations of probable landslide dams near the Kes Gah area of Chillas, along the Indus River, which are likely associated with the knickpoints, shown as red and yellow dots (with their respective ksn values).

map also shows the locations of probable landslide dams and associated blockages of the channel. There are many other examples of mapped landslides and faults exhibiting spatial correlations with knickpoints across the study area. A few of them are provided on the Figure 13. These comparisons reveal that moderate to high ksn values were observed at historically documented debris dam sites, as well as interpreted landslide/rockslide debris dam sites of unknown ages, especially the GolGhone (ksn = 877 m0.9 ), Katzarah (ksn = 1,610 m0.9 ), 10

Lichar Gah (ksn = 2,951 m0.9 ), and Kes Gah (ksn = 1,278 m0.9 ) historic events. The knickpoints observed adjacent to these historic slides suggest the likely correlation between mapped landslide dams and knickpoints, in the absence of any noticeable lithologic change or known faults. At a few locations, the knickpoints exhibited very high ksn values (between 1,800 and 4,200 m0.9 ). These tend to occur where the breached debris dams overlap with the active faults crossing the channel (especially in the NPHM region), which, in turn, highlights the composite role of landslide dams, uplift, and active faulting in knickpoint formation. The landslide inventory map (Ahmed and Rogers, 2014) included 451 mapped landslide features along both sides of the Indus River, in a corridor approximately 10 km wide (i.e., around 5 km along either side of the river). Close screening of the inventory map revealed that 337 landslides out of the 471 mapped slides extended to the banks of the Indus River. The other slides are either small, or their toes terminate more than 1 km from the river. This suggests that 337 landslides likely impacted the Indus River channel. In this way, the gross comparison of landslides to knickpoints (Table 1) suggests that 193 of the 337 (or 57 percent) mapped landslides appear to be spatially associated with knickpoints, while 28 of the 39 (72 percent) documented prehistoric and historic landslide dams, reported by other researchers in the study area (Hewitt, 1982, 1998, 2002, 2009; Shroder and Bishop, 1998; Korup et al., 2010; and Hewitt et al., 2011), also appear to exhibit marked knickpoints.

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Figure 13. Rockslide avalanches likely blocked the Indus River at different locations. (A) Excerpts of landslide inventory a few kilometers upstream of Dasu in the Pani Ba area. (B) Upstream of Char Nala (Dasu), showing the locations of knickpoints likely linked with active faulting (running along the river) and mapped landslides. (C) Probable landslide blockage along the Indus River near Thakot, where the knickpoints are probably influenced by active faults (running along the river) and mapped landslides. (D) Landslide inventory map in vicinity of a probable landslide dam (red stipple) near Bulder Gah, along the Indus River. The linear cyan line is a mapped fault cutting through the area.

Figure 14. Normalized relationship between mapped landslides and number of knickpoints with geologic map units. These histograms summarize knickpoint data normalized over 50 km of Indus River profile length.

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Ahmed, Rogers, and Ismail Table 1. Summary of observed knickpoints and frequency of related landslides. Description

No.

Calculation

Mapped landslides Mapped landslides exhibiting knickpoints

451 193

Historic landslide dams Historic landslides exhibiting knickpoints

39 28

(193/451) × 100 = 43% of the observed landslides appear to be associated with identified knickpoints. (28/39) × 100 = 72% of documented landslide dams exhibit spatial agreement with identified knickpoints.

Normalized Relationships among Knickpoints, Lithology, and Landslides A gross comparison of knickpoints and landslide dams with underlying lithologic units was also performed to investigate any relationship between these geomorphic features. Figure 14 shows the normalized relationship between mapped landslides and the major geologic units. The massive bedrock units, including Tkm, Tkb, Pct, and Eg, are associated with the least number of slides. The bedrock correlations suggest that the areas associated with massive landslides are those of the more highly weathered gneisses, amphibolites, and graphitic blueschist and greenschist of the Indian Basement Complex of Precambrian age, the HazaraKashmir Basement Complex (PCb-PCs), undifferentiated metasedimentary rocks (MPzm), and Mesozoic to Paleozoic rocks of the Northern Suture M´elange (MPzs). Mica schist terrains are particularly susceptible to triggering large bedrock landslides, likely because of the low friction developed along micaceous planes of foliation and their tendency to retard pore pressure dissipation, especially during translatory movement (Morton and Sadler, 1989). Figure 14 shows the majority of knickpoints were observed in the rock units MPzm, MPzs, PCb, and PCs. Overall, these units are composed of materials with medium to low strength parameters. The likely correlation between knickpoints and prehistoric landslides in the NPHM region and the prehistoric Gol-Ghone, Katzarah, and historic Lichar Gah (1841) landslide dams suggests that the majority of observed knickpoints appear to correlate well with landslide dams, which highlights the importance of landslide dams as a significant factor in knickpoint creation. This does not diminish the significance of regional tectonics, active faults, and changes in bedrock lithology and bedrock incision as additional

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significant factors in triggering knickpoints along the channel. CONCLUSIONS This study was conducted to examine whether knickpoint analysis using publicly available data and analytical tools could be used to identify the presence of significant landslide dams on the Indus River. The knickpoints and their related geomorphic parameters (channel profile, concavity, drainage area, and normalized steepness index, etc.) were extracted from the analysis of ASTER GDEMs with 30 m resolution, using ArcGIS and Matlab software. In total, 251 major and minor knickpoints were extracted from the river’s longitudinal profile, extending approximately 750 km upstream of Tarbela Dam. The knickpoint locations and their respective normalized steepness index (ksn ) values were compared with the regional-level landslide inventory maps and the locations of historic rockslides. These analyses suggest that correlations can be drawn between spatial locations of knickpoints and mapped and documented landslides dams. This correlation is reflected in moderate to high normalized steepness index (ksn ) values at several landslide debris dam sites, particularly in the Gol-Ghone (ksn = 877), Katzarah (ksn = 1,610), Lichar Gah (ksn = 2,951), and Kes Gah (ksn = 1,278) areas. Gross comparison between knickpoints and landslides shows that 57 percent of the mapped landslides and 72 percent of the historic landslide dams impacting the channel are sufficiently pervasive to have formed recognizable knickpoints. These geomorphic observations and comparisons appear to validate the concept that rock slope failures have impacted the Indus River’s longitudinal profile on multiple occasions in the geologic past. This study further concluded that the highly complex and irregular longitudinal profile of the Indus River has likely resulted from the complex interaction of various geomorphic processes, and that landslide dams are one of the significant mechanisms by which knickpoints form in the channel. Using only publicly available data analysis, it is difficult to attribute each knickpoint to a definite cause, such as landslides or other possible causes of knickpoints, but this analysis demonstrates that there are numerous locations along the channel where a better correlation exists between knickpoints and landslide damming processes. More detailed site-specific analyses aided by higherresolution data would need to be undertaken to verify the assumptions used for this investigation, which focused mainly on identification of anomalous topography (identifying landslide features) and perturbations of the channel profile (identifying knickpoints).

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Further studies would be required to differentiate knickpoints triggered by regional tectonic uplift, local fault offset, bedrock erodibility, or landslide/rockslide dams. ACKNOWLEDGMENTS The authors wish to thank the Natural Hazards Mitigation Institute at the Missouri University of Science and Technology, Rolla, MO, USA, and the Center of Excellence for Geospatial Information Science of the U.S. Geological Survey in Rolla, MO, for providing the opportunity to accomplish this work. The authors would also like to thank University of Engineering and Technology, Lahore, Pakistan, for financial support to one of the authors to conduct this research. REFERENCES AHMED, M. F. AND ROGERS, J. D., 2012, Landslide mapping and identification of old landslide dams along the Indus River in Pakistan, using GIS techniques. In Proceedings of the AEG 55th Annual Meeting, Vol. 55: Salt Lake City, UT, Association of Environmental & Engineering Geologists, 44 p. AHMED, M. F. AND ROGERS, J. D., 2013, Thalweg profiles and knickpoints as useful discriminators of prehistoric landslide dams in northern Pakistan: Geological Society of America Abstracts with Programs, Vol. 45, No. 7. AHMED, M. F. AND ROGERS J. D., 2014, Creating reliable, firstapproximation landslide inventory maps using ASTER DEM data and geomorphic indicators, an example from the upper Indus River in northern Pakistan: Environmental & Engineering Geoscience, Vol. 20, No. 1, pp. 67–83. AHMED, M. F.; ROGERS, J. D.; AND ISMAIL, E. H., 2014, A regional level preliminary landslide hazard study of upper Indus River basin: European Journal of Remote Sensing, Vol. 47, pp. 343– 373. AHMED, M. F.; ROGERS, J. D.; AND ISMAIL, E. H., 2017, Knickpoints and various geomorphic processes along the upper Indus River, Pakistan: Swiss Journal of Geosciences (under review or accepted). BISHOP, P.; HOEY, T. B.; JANSEN, J. D.; AND ARTZA, I. L., 2005, Knickpoint recession rate and catchment area: The case of uplifted rivers in eastern Scotland: Earth Surface Processes and Landforms, Vol. 30, pp. 767–778. CODE, J. A. AND SIRHINDI, S., 1986, Engineering implications of the impoundment of the Indus River by an earthquake induced landslide. In Schuster, R. L. (Editor), Landslide Dams: Processes, Risk and Mitigation: Geotechnical Special Publication No. 3, American Society of Civil Engineers, Reston, VA, pp. 97–110 CROSBY, B. T.; WHIPPLE, K.; GASPARINI, N. M.; AND WOBUS, C. W., 2007, Formation of fluvial hanging valleys; theory and simulation: Journal of Geophysical Research, Vol. 112, pp. F03S10. CROSTA, G. B., 2001, Failure and flow development of a complex slide: The 1993 Sesa landslide: Engineering Geology, Vol. 59, pp. 173–199. CRUDEN, D. M. AND VARNES, D. J., 1996, Landslide types and processes. In Turner, A. K. and Schuster, R. L. (Editors), Landslides, Investigation and Mitigation: Special Report 247, Transportation Research Board, Washington, D.C., pp. 36–75.

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