Hydrological connectivity of a Danube river-floodplain ...

Hydrological connectivity of a Danube river-floodplain system in the Austrian Machland: changes between 1812 and 1991 Modifications de la connectivité hydrologique d’une plaine alluviale du Danube dans le Machland autrichien entre 1812 et 1991 Severin Hohensinner 1*, Gregory Egger 2, Gertrud Haidvogl 1, Mathias Jungwirth 1, Susanne Muhar 1, Stefan Schmutz 1 1

Institute of Hydrobiology and Aquatic Ecosystem Management, Department of Water, Atmosphere and Environment Max Emanuel-Str. 17, A-1180 Vienna, Austria. University of Natural Resources and Applied Life Sciences Vienna 2 Institute for Ecology and Environmental Planning, Bahnhofstr. 39, A-9020 Klagenfurt, Austria * Correspondence to: Severin Hohensinner, University of Natural Resources and Applied Life Sciences Vienna Institute of Hydrobiology and Aquatic Ecosystem Management, Max Emanuel-Str. 17, A-1180 Vienna, Austria E-mail: [email protected], tel.: ++43 1 47654 5209

Abstract The ecological integrity of alluvial river landscapes is largely determined by hydrological connectivity - the exchange processes between river and floodplain. Historical surveys of the Danube River in the Austrian Machland enable the assessment of natural connectivity conditions prior to channelization. In 1812, active overflow and seepage inflow of nutrient-rich Danube water prevailed in the floodplain, favoring high rates of primary production. Following channelization and flow regulation up to 1991, hydrological connectivity drastically decreased. Together with the strong reduction of eupotamal water bodies, the Danube river-floodplain ecosystem experienced a substantial change towards prolongated disconnection phases, resulting in groundwater-fed, low/medium productive conditions. Keywords: Danube River, alluvial floodplain, hydrological connectivity, flow pulse, seepage inflow, active overflow, historical change

Résumé L’intégrité des paysages alluviaux est fortement déterminée par la connectivité hydrologique, c’est-à-dire les échanges entre le fleuve et sa plaine d’inondation. Des études historiques du Danube dans le Machland autrichien ont permis d’établir les conditions de connectivité naturelle avant la canalisation. Les débordements et les infiltrations d’eau du Danube riche en nutriments se produisaient dans la plaine alluviale en 1812, favorisant alors une production primaire élevée. En 1991, avec la canalisation et la régulation des flux, la connectivité hydrologique a nettement diminué. Les écosystèmes alluviaux danubiens ont subi des changements drastiques impliquant des phases de déconnexion prolongées qui ont eu pour résultat une perte de productivité. Mots clés : Danube, plaine alluviale, connectivité hydrologique, fluctuation des écoulements, infiltrations, débordements, changement historique

© 2007 Lavoisier SAS. Tous droits réservés

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Fonctionnement naturel des zones alluviales et conséquences des aménagements des fleuves

Version abrégée La connectivité hydrologique, traduite par les échanges entre le fleuve et sa plaine d’inondation et la pulsation hydrologique, c’est-à-dire l’expansion/contraction des surfaces en eau dans l’espace et dans le temps jouent un rôle majeur dans le fonctionnement naturel des écosystèmes alluviaux (Amoros & Roux, 1988; Ward & Stanford, 1995a; Jungwirth et al., 2000). Les fluctuations des écoulements «flow pulse» en dessous des niveaux plein bord (Puckridge et al., 1998) et les fluctuations des niveaux de crue «flood pulse» au dessus des niveaux plein bord (Junk et al., 1989) non seulement stimulent les échanges, mais aussi fournissent des habitats pour les organismes vivants en changement permanent (Tockner et al., 1999; Ward & Tockner, 2001). La présente étude analyse dans un premier temps la connectivité hydrologique du Danube dans la zone alluviale du Machland autrichien sur la base d’études historiques de 1812. Dans un deuxième temps, les changements du régime hydrologique liés à la rectification et la construction d’une usine hydroélectrique sont discutés. Le site d’étude est un tronçon du Danube de 10,25 km de long localisé dans l’est du Machland (point kilométrique 2094-2084) le long de la frontière entre la basse Autriche et la haute Autriche (Figure 1, Table I). En 1812, le lit du Danube était occupé par des îles végétalisées et des bancs de graviers, le chenal principal et les bras latéraux dominant le paysage alluvial (Hohensinner et al., 2004; Figures 2, 3a-b). Les premiers aménagements pour la navigation ont débuté en 1826 et la phase majeure de la rectification fut terminée en 1859. Aujourd’hui, le tronçon étudié concerne la tête du réservoir de l’usine hydroéléctrique Ybbs-Persenbeug ; la plupart des plans d’eau de la plaine alluviale est isolée du chenal principal par des digues. Pour cette étude la carte des données hydromorphologiques de 1812 a permis de décrire l’état du fleuve avant la rectification. Un modèle numérique de terrain en 3 dimensions a été réalisé pour définir les conditions de fluctuation des écoulements et de la connectivité des années 1812 (état original) et 1991 . La modélisation de la connectivité révèle que les fluctuations du niveau d’eau en dessous du niveau plein bord sont très significatives. Les fluctuations des basses eaux (“low flow pulse” LFP) correspondant aux fluctuations entre les basses eaux (LW) et les niveaux d’eau estivaux (SMW) modifient de manière permanente les habitats aquatiques

1. Introduction Hydrological connectivity - the intensive exchange processes between the river and the floodplain - has become recognized as a central aspect for natural functioning of alluvial floodplain ecosystems (Amoros & Roux, 1988; Ward & Stanford, 1995a; Ward & Tockner, 1999; Jungwirth et al., 2000). The “pulse” © 2007 Lavoisier SAS. Tous droits réservés

dans le lit minéral (Table II and III). Les fluctuations des hautes eaux (“high flow pulse” HFP) entre les niveaux d’eau estivaux SMW et le niveau plein bord (BW) augmentent significativement la connectivité latérale en inondant de larges surfaces végétalisées (e.g. surface de végétation pionnière, petites îles, chenaux abandonnés végétalisés), ainsi les habitats aquatiques se sont développés dans des régions parfois éloignées du lit mineur dans la plaine alluviale (Figures 4a-h, 5). En conséquence, au niveau plein bord, 57 % de la zone active (chenaux et plaine inondable) étaient inondées. A cause d’une connectivité hydrologique importante, le niveau de la nappe reste élevé, à environ 1,6m sous la surface du sol, même aux niveaux d’eau moyens (Figure 6). La canalisation et la construction de centrales hydroélectrique ont perturbé la dynamique fluviale, en diminuant la connectivité hydrologique. En 1991, la surface des plans d’eau a diminué de 54% aux niveaux estivaux SMW et le niveau de la nappe a subi une chute notable. Les fluctuations des basses eaux LFP sont passées de 17 % de la zone active en 1812 à 1 % en 1991 (Figure 5, Table III). Les fluctuations des hautes eaux HFP sont passées de 13 % à 11 %, dont en 1991 seul 1 % provenait du débordement du Danube et 10 %, localisés dans les secteurs isolés, provenaient des flux de versant et des infiltrations d’eau du Danube. En 1812, les fluctuations des basses eaux résultaient probablement des infiltrations d’eau du Danube riche en nutriments, sur de longue durée et sur de larges surface, ce qui favorisait des taux de production primaire élevés (Heiler et al., 1995; Hein et al., 1999a, 1999b; Tockner et al., 2000a, 2000b; Amoros & Bornette, 2002). En 1991, les fluctuations des basses eaux (LFP) ont été fortement réduites et sont principalement contrôlées par les apports souterrains. En période de hautes eaux, les débordements jouent un rôle majeur aussi bien en 1812 qu’en 1991. En 1812, les inondations résultaient des débordements du Danube. En 1991, les fluctuations des hautes eaux dépendent principalement des inondatins à partir de petits bras ou fossés. Les écoulements de versant en subsurface et les infiltrations du Danube contribuent à la connectivité dans les zones isolées de la plaine d’inondation. Outre la réduction drastique des plans d’eau du potamon jusqu’en 1991, les changements de la connectivité induites par les activités humaines ont fortement altéré les écosystèmes alluviaux du Machland : la production primaire élevée a évolué vers une production actuelle faible à moyenne.

of the discharge and, consequently, the spatial and temporal expansion/contraction of the water surface plays a crucial role. “Flow pulse” below bankfull (Puckridge et al., 1998) and “flood pulse” above bankfull (Junk et al., 1989) not only stimulate diverse exchange processes, but also provide a permanently changing habitat spectrum for aquatic organisms (Tockner et al., 2000a, 2000b; Ward & Tockner, 2001).

Hydrological connectivity - Danube River

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Fig. 1: Location of the study area in the eastern Machland, Upper/Lower Austria.

The present study, in a first step, surveys and analyses hydromorphological data of the Danube River in the alluvial zone of the Austrian Machland based on historical references from 1812 (Hohensinner et al., 2004). The primary goal is to describe the character and significance of former hydrological connectivity within a natural river-floodplain system that was typical for the Austrian Danube River prior to river straightening. The focus is on how hydrological connectivity and, accordingly, the exchange pathways of water, sediments, nutrients and organisms were influenced by different water levels within the floodplain. This approach views the floodplain as an integral part of a multi-dimensional fluvial hydrosystem with various lateral interactions between the river and the surroundings (Amoros et al., 1987; Junk et al., 1989; Ward, 1989; Naiman & Décamps, 1990, 1997; Stanford & Ward, 1993; Ward & Stanford, 1995b; Petts & Amoros, 1996; Jungwirth, 1998; Ward & Wiens, 2001). In a second step, changes of the natural hydrological connectivity induced by river straightening and hydropower plant construction are discussed. The results and conclusions of these investigations are designed to serve as valuable baseline data in the sense of river-type-specific reference conditions (“Leitbild”) © 2007 Lavoisier SAS. Tous droits réservés

for future mitigation and restoration measures in similar reaches of the Danube River (e.g. in the Austrian Danube Floodplain National Park) (Petts et al., 1989; Kern, 1992a, 1992b; Muhar, 1994; Muhar et al., 1995; Jungwirth et al., 2002). 2. Study site The study site is a 10.25 km-long section of the Danube River located in the eastern Machland (river-km 2094-2084) along the border of Upper and Lower Austria (Fig. 1). The so-called Machland is the most eastern of three tectonic Danube basins in Upper Austria, which are separated by the Bohemian Massif. It is strongly influenced by three large alpine tributaries (Inn, Traun and Enns), which are all rich in bedload and suspended sediment load (HZB, 1937; UNESCO, 1993). Since these tributaries are interrupted by chains of dams, bedload transport is blocked today. Discharge is mainly influenced by alpine flow conditions and it peaks in spring and summer due to the snowmelt in the Alps (Mader et al., 1996). The study area coincides with

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Fig. 2: Study area overview prior to river straightening in 1812; dotted line: border of the study area (10-year flood area); grey areas: active zone (includes water bodies, unvegetated gravel/sand areas, vegetated islands and morphologically young floodplain sections that were formed by hydrological conditions since 1500 A.D.); black areas:

water bodies inside the active zone as mapped in 1812 at slightly increased low water level (LW+); long, narrow, enclosed shapes within the floodplain are abandoned vegetated channels that are dry at water levels between LW and approx. SMW; (compare Figures 3a and 3b for transects).

Table I: Characteristics of the Danube River in the eastern Machland, Austria. Stream order 1 Channel slope in 1812, river-km 2094-2084 (m m-1) 2 Low annual discharge 1961-1990, river-km 2094 (m3 s-1) 3 Mean annual discharge 1961-1990, river-km 2094 (m3 s-1) 3 Mean annual flood discharge 1951-1990, river-km 2094 (m3 s-1) 4 10-year flood 1954-1991, river-km 2094 (m3 s-1) 5 Mean annual bedload before 1850, river-km 2060 (m3 a-1) 6 Mean annual bedload 1987-1998, river-km 2094-2084 (m3 a-1) 7 Mean annual suspended load 1928-1936, river-km 2110 (t a-1) 8 Mean annual suspended load 1982-1991, river-km 2094 (t a-1) 9 Mean grain size in 1937, river-km 2084 (mm) 10 Mean grain size in 1961, river-km 2084 (mm) 11

9 0.00055 860 1800 ca. 5800 7300 490,000 * 6,950,000 2,750,000 20 22

* only local transport within the head of the impoundment and during floods between the impoundments 1 Wimmer & Moog, 1994; 2 river survey map of the k.k. Landesbaudirection, 1812; 3 WSD (Austrian Federal Waterway Agency), 1998; 4 calculated based on Amt der oö. Landesregierung, 1994; HZB, 1980, 1981, 1984, 1995; 5 Amt der oö. Landesregierung, 1994; unpublished data of the WSD, 2001; 6 Schmautz et al., 2000; 7 Schimpf & Harreiter, 2001; unpublished data of river bed surveys of the WSD, 1987-1998; 8 calculated based on HZB, 1937; 9 calculated based on unpublished data of the AHP, 1982-1991; 10 HZB, 1937; 11 Schmutterer, 1961; UNESCO, 1993

present 10-year flood area, which is delimited to the north by the terrace of the Würm glaciation and to the south by the Tertiary hill country. In 1812 this area covered 33.8 km², of which 22.2 km² (66 %) belonged to the active zone (Fig. 2). In this context the active zone (AZ) includes the active channel system (water bodies and unvegetated gravel/sand areas), vegetated islands and morphologically young flood© 2007 Lavoisier SAS. Tous droits réservés

plain sections that were formed by hydrological conditions of Modern times (since approx. 1500 A.D.). Originally, the AZ was partially flooded at mean annual flood, and total inundation occurred every 3-5 years. The remainder of the study area (10-year flood area) is formed by an older and therefore partly higher alluvial terrace, the so-called “lower postglacial valley floor” (sensu Kohl, 2000). It aggraded

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Hydrological connectivity - Danube River Table II: Characteristic water levels of the Danube River in 1812 and comparable discharges in 1991 (eastern Machland, river-km 2094-2084). Characteristic water level Abbreviation

Water level (m above LW)

Discharge in 1991 (m3 s-1)

Low water level LW 0.00 860 (zero point of gauge 1812) Low water level + LW+ 0.30 * (water level of survey 1812) Mean annual water level MW 1.30 1800 Summer mean water level SMW 1.70 2150 Bankfull water level (average) BW 2.00-3.50 1 4500-5000 Flood (return period = 3-5 years) HW3-5 3.20-7.60 2 ca. 6500 * no data available 1 Diverging levels of the Danube water table at BW due to the backwater effect of the narrow Danube canyon section downstream of the study area. The minimum value refers to the water level at the upstream end of the study site, the maximum value to the downstream water level (compare Fig. 5). The average level is 2.50 m above LW (calculated as weighted average of area shares based on the DTMs). 2 Diverging levels of the Danube water table at HW3-5. Weighted average level = 5.30 m above LW.

during Roman times or in the Early/High Middle Ages and is sparsely populated by humans (Kohl, 1990). Because of terrain depressions within this older terrace, some sections show elevations similar to the AZ (Fig. 3a, 3b). Flooding occurred here with return periods between 2 and 10 years. On average the width of the whole study area (10-year flood area) is 3200 m, that of the active zone 2100 m. Owing to the narrow Danube canyon section directly downstream of the studied river section, substantial backwater effects occur during floods in the downstream-located areas of the study site (Gruber, 1960). In 1812, the Danube River channel system in the Machland was branched by several vegetated islands and gravel bars (Fig. 2). The river landscape was characterized by eupotamal water bodies (main channel and side arms), offering a primarily lotic environment (Hohensinner et al., 2004). As recorded in 1812, the mean total width of the channel system was 550 m at the low flow situation and 730 m at summer mean flow. The main channel was clearly recognisable, however, and along some reaches was split into two morphologically similar anabranches. According to the river typology of Nanson & Knighton (1996) based on the classification of Brice (1984), the studied Danube River reach can be designated as a graveldominated, laterally active anabranching river. The first river straightening measure along this Danube reach was a training wall constructed around 1826. In 1859 the first major phase of river straightening was completed. In the 20th century the riverine landscape was subject to further massive changes caused by the construction of two Danube hydropower plants, Ybbs-Persenbeug (1957, 23 km downstream) and Wallsee-Mitterkirchen (1968, at the upstream border of the study area, Fig. 4e). Today, the investigated Danube reach is the head of the reservoir of the power plant Ybbs-Persenbeug (Fig. 1). Most floodplain waters were separated from the main channel by dikes. Two pumping stations, located at the confluences of two dominating backwater systems to the main channel, drain the floodplain for the purpose of lowering the groundwater table (Fig. 4e). Table I shows some characteristic physical parameters of this Danube River reach. © 2007 Lavoisier SAS. Tous droits réservés

3. Data sources and methodology From 1714 onwards, exact surveys of the riverine landscape were conducted in order to determine property borders. Moreover, plans to improve navigation were initiated early, giving rise to a series of detailed river maps of this Danube stretch from 1812 onwards. 120 historical maps (land surveys, estate maps, military-topographical surveys, river surveys, navigation maps, …) of this river reach have been found in various Austrian archives. Gathering the most accurate ones, 45 selected maps were superimposed over current detailed topographical surveys using AutoCAD Overlay. Planform accuracy was checked by means of benchmarks (churches, streets, terrain structures, ...) that have remained unchanged since their mapping. For this study, the most accurate map (river survey mapped by the former k.k. Landesbaudirection, scale 1 : 6900; Provincial Archive of Upper Austria) dating to 1812 was selected to describe the former natural state of the riverine landscape. It comprises information about terrain elevation and characteristic water levels (measured as spot heights), flow velocities and vegetation. A supplementary longitudinal profile shows water depths, the river bottom along the thalweg and several water surface gradients. In order to eliminate planform inaccuracies, the map was digitised, geometrically corrected using the benchmarks and vectorised. Table II shows the characteristic water levels in 1812. In this study the topographical reference point is the zero point of gauge in 1812, corresponding to the low water level (LW) in 1812. Though some data on the discharge during the 19th century exist, they are not very reliable and therefore characteristic discharges from the 20th century are used. The current situation of the investigated Danube River reach can be easily assessed based on various maps and surveys (e.g. Austrian Map ÖK 25, scale 1 : 25000; Carte de Pilotage du Danube, WSD, 1992, 1 : 10000; aerial photograph interpretations, AHP, 1 : 2000). Hydrological data regarding surface waters and groundwater conditions as well as bathymetric surveys are provided by WSD 1941-1998, federal and provincial organisations (e.g. Breiner, 1976; Amt der oö. Landesregierung, 1994) and AHP 1963 (compare Table I).

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Hydrological connectivity - Danube River Figures 3a-b: Transects of the study site at river-km 2089,45 (a) and 2088,70 (b): terrain surfaces, water bodies and groundwater table at mean water in 1812 and 1991. MW = mean water, MGW = groundwater table at mean water (compare Figure 2 for location of the transects within the study area).

First results regarding the hydromorphological conditions in 1812 were gathered by analysing the historical surveyed spot heights of the terrain and the water levels (Hohensinner et al., 2004). For this study, three-dimensional digital terrain models (DTMs) were generated in form of triangulated irregular networks (TINs: models based on triangles) using the CAD/GIS-program Autodesk Land Desktop for both the situations in 1812 and 1991. One of the great advantages of these vector-based models is that breaklines of the terrain surface as well as shorelines can be accurately edited. In a first step, the spot heights of the terrain surface and the water surfaces at different stages that are mapped in the cartographic sources were used to build TINs for each of these surfaces. In order to estimate the level of the groundwater table between the channels in the floodplain in 1812, the mapped spot heights of the water surfaces were interpolated and incorporated into the TINs. In a following work step, water cover at different water levels was calculated based on the intersection of TINs of the

terrain surface and the specific water/groundwater surfaces. Additionally, depths of the groundwater table in relation to the terrain surface were computed by the same method. The generated data enable conclusions to be drawn on the “flow pulse” and connectivity conditions of the original riverfloodplain system. Connectivity can be assessed by means of the water-covered area (expressed as a percentage) of the AZ at a given flow. The “flow pulse” is evaluated based on the expansion/contraction of the water surface area referring to the AZ (Table III). In order to estimate the influence of the “flow pulse” within the river landscape at specific stages, we differentiate between “low flow pulse” (LFP), which occurs at water level fluctuations between low water (LW) and summer mean water (SMW) within the gravel-bed of the river, and “high flow pulse” (HFP). The HFP refers to fluctuations between SMW and bankfull water (BW) and affects low-lying vegetated sites like recently aggraded areas with pioneer vegetation, small islands and abandoned channels that are now vegetated. At BW, flooding does not exceed the bank slopes and occurs only in the deeper, partly vegetated areas of the floodplain. While LFP and HFP are restricted to the lower terrain elevation zones, the “flood pulse” above BW also affects the elevated areas of the AZ or - depending on the flood stage - even the whole study area (= 10-year flood area).

Table III: Water cover at characteristic water levels and expansion of the water surface related to the surface of the original active zone (AZ) in 1812 (%). Water level

1812 Expansion

Cov. (%)

1991 Exp. (%)

Cov. (%)

LW 27 19 LW - LW+ 6 LW+ 33 19 LW+ - MW 8 MW 41 20 MW - SMW 3 SMW 44 20

Exp. (%)

0.1 0.5 0.2

}

LFP

HFP

SMW - BW 13 BW 57 31 1

11 1

BW - HW 43 HW 100 85

54

Flood pulse

Total expansion

LW - BW

30

12

Flow pulse

Total expansion

LW - HW

73

66

Total pulse

Cov. = water cover of the AZ at the indicated water level (%), Exp. = expansion of water surface between the indicated water levels (%), LW = low water, LW+ = slightly increased low water (as mapped in 1812), MW = mean water, SMW = summer mean water, BW = bankfull water, HW = flood (1812: return period = 3-5 years, 1991: 4 years), LFP = low flow pulse (water level fluctuations between LW and SMW), HFP = high flow pulse (between SMW and BW)

only 1 % derives from Danube active overflow, 10 % in isolated floodplain terrain derive from groundwater inflow from the hillslope aquifer, inflow of some smaller brooks and Danube seepage water that seeps through the dikes 1


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(a)


Figures 4a-h: Water surface expansion within the Danube river-floodplain system before river straightening in 1812 (a-d) and after river straightening and hydropower plant construction in 1991 (e-h). (a) 1812: water surface at low water, (b) 1812: summer mean water. Black areas: water surfaces; grey areas: unvegetated gravel and sand; white areas: vegetated terrain;

(b)


(c)

(c) 1812: bankfull water, (d) 1812: 3-5-year flood.

(d)



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(e)


(e) 1991: low water, HP = hydropower plant Wallsee-Mitterkirchen at the upstream end of study river section; PS = pumping stations for floodplain draining at the downstream end of study section; the white linear structures in the main channel are training walls and groins for low flow regulation. (f) 1991: summer mean water.

(f)


(g)

(g) 1991: bankfull water, (h) 1991: 4-year flood.

(h)



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4. Results 4.1. Hydrological connectivity in 1812 In 1812, 27 % of the AZ was flooded at LW and 44 % at SMW. Thus, 17 % of the AZ were directly affected by frequent fluctuations of the LFP, whose amplitude averaged 1.7 m (Fig. 4a, 4b and 5). Further expansion of the water surface area at water levels between SMW and BW (HFP) affected an additional 13 % of the AZ (Fig. 4b, 4c and 5). Dependent on the magnitude of the HFP, the surface connectivity spread from the main channel over the AZ to far over the “lower postglacial valley floor”, enabling lateral connections to habitats more than 2 km away from the main channel. The rapid and large surface expansion of the HFP was mainly facilitated by the dense network of abandoned vegetated channels. In 1812, these channels amounted to approximately 240 ha (23 ha/km valley axis) within the study site, 210 ha of them featuring intact connections to the main channel or nearby floodplain water bodies (Hohensinner et al., 2004). Hence, total water coverage at BW amounted to 57 % of the AZ, with the whole “flow pulse“ between LW and BW totalling 30 %. Due to the narrow Danube canyon section directly downstream of the study site (Fig. 2), backwater effects occurred at flows above SMW (Gruber, 1960). As a consequence, above SMW, the Danube water table rose from the upstream end of the study site towards the downstream end at the entree of the canyon section (compare Table II, Fig. 5). Finally, when water level rose above BW, the whole floodplain was gradually inundated. During floods with return periods of 3-5 years the total AZ was water-covered, the “flood pulse” affecting the remaining area of the floodplain (43 % of the AZ, Fig. 4d). The digital terrain models (DTMs) also yielded estimates of former groundwater table levels in the floodplain at given flows. Figure 6 shows the terrain area shares for particular groundwater table depths at mean water (MW) in the AZ (based on half-meter classes). In 1812, the middle 50 % of the groundwater depths approximately referred to the depthclasses of 1.0 to 2.0 m. On the average the groundwater table was only 1.6 m beneath the terrain surface within the AZ (calculated as weighted average related to area shares). The modelled levels of the water/groundwater surfaces at MW allow the assumption that in some reaches the water table of the river was up to ca. 1 m higher than the groundwater table of the adjacent floodplain (Fig. 3a, 3b). This was probably due to a large island in the center of the river system, which hindered runoff in the main channel, and the relatively high main channel bottom downstream of that island (Fig. 2). The spot heights of the water surface surveyed at MW in 1812 show a significant reduction of the surface slope from 0.00047 m m-1 (mean slope at MW of the studied river reach) down to 0.00003 m m-1 in the 3500 m long river reach upstream of the central island. This indicates a substantial rise of the water table due to the backwater-effect of the island. Based on the modelled water/groundwater surfaces at MW, we conclude that © 2007 Lavoisier SAS. Tous droits réservés

almost the whole AZ exhibited groundwater table gradients between the river channels or from the channel system to the adjacent floodplain aquifer (based on the assumption of a longlasting constant mean flow situation when the flow regime of Danube River largely controls the groundwater table in the floodplain; Fig. 3a, 3b). Accordingly, 84 % of the total study site (10-year flood area) featured such hydrological conditions. Only remote areas of the study site far away from the Danube channel system showed reverse groundwater slopes from the hillslope aquifers of the adjoining hinterland (Würm terrace in the north, Tertiary hill country in the south) and from some smaller tributaries towards the Danube River. Similar results are also computed for the low flow situation (LW), with minor groundwater table gradients between the Danube River and the floodplain occurring in approximately 69 % of the total study area (almost 100 % of the AZ). In 1812, therefore, at low flow and mean flow a persistent seepage inflow from the Danube River to the floodplain aquifer and floodplain water bodies must be assumed. 4.2. Hydrological connectivity in 1991 Up to 1991, dikes that were created during river straightening and hydropower plant construction cut off backwaters and separated active and abandoned channels from the main channel. The water surface area decreased by 44 % at LW (54 % at SMW) and the lateral exchange processes characterising the original system were nearly completely interrupted. In 1991, at LW 19 % of the AZ was water-covered and the LFP was significantly reduced affecting only ca. 1 % of the AZ (Table III, Fig. 4e, 4f and 5). Accordingly, the total submerged area amounted to only 20 % at SMW. Though the HFP affected 11 % of the total AZ, only 1 % derived from active overflow of Danube river water. The remaining 10 %, located in isolated floodplain terrain, were inundated by subsurface inflow from the hillslope aquifer of the hinterland, inflow of some smaller tributaries and Danube seepage water due to the separation of the floodplain from the main channel by the dikes. When water level rose up to BW, flooding added up to 31 % of the AZ (Fig. 4g). Summarizing the changes between 1812 and 1991, the total “flow pulse” affected area at fluctuations between LW and BW significantly decreased from originally 30 % of the AZ to 12 %, whereby only 2 % derived from Danube active overflow in 1991. During 4-year floods - comparable to the 3-5-year flood measured in 1812 - 85 % of the AZ were inundated in the 1991 situation (Fig. 4h). This resulted in 54 % of the AZ being affected by the “flood pulse”. Since the construction of the hydropower plants, total inundation of the AZ occurs at floods with return periods of 10 years. Strongly decreased connectivity drastically reduced typical floodplain habitats and led to a substantial drawdown of the groundwater table. In 1991, the mean depth of the groundwater table almost doubled compared to 1812 (weighted average in 1991 = 3.0 m, Fig. 6) and the middle 50 % of the groundwater depths in the AZ were approximately represented


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the formation of perennial vegetation. Although some floodplain water bodies were separated from the Danube River at low flow, approximately 97 % of the overall water surface offered a primarily lotic environment (main channel and side arms connected on both ends; Hohensinner et al., 2004). The intensive hydrological connectivity due to the LFP is also reflected by the relatively high groundwater table in the former floodplain (Fig. 6). Water level fluctuations between SMW and BW (HFP) significantly extended lateral connectivity by inundating large low-lying vegetated areas, thereby expanding Figure 5: Flooded area of the active zone (%) in relation to characteristic water levels in 1812 and 1991. aquatic habitats to remote regions LW = low water, LW+ = slightly increased low water, MW = mean water, SMW = summer mean water, of the floodplain. Overall, this BW = bankfull water, HW = flood (1812: return period = 3-5 years, 1991: 4 years);. high degree of connectivity was BW and HW3-5 in 1812: diverging levels of the Danube water table due to the backwater effect of the narrow not only attributed to the active Danube canyon section downstream of the study area. The minimum value refers to the water level at the upstream end of the study site, the maximum value to the downstream water level. Average water level at channels but also to the large BW = 2.50 m above LW and 5.30 m at HW3-5 (calculated by means of DTMs). At HW3-5 maximum water network of abandoned vegetated level peaks 7.60 m above LW. BW and HW4 in 1991: water levels are additionally affected by the flow channels. These morphologically regulation of the hydropower plant. Average water level at BW = 4.10 m above LW and 4.60 m at HW4. older floodplain elements served to interconnect the younger river by the depth-classes of 2.0 to 3.5/4.0 m. These changes in channels and the various floodplain sections when water hydrological connectivity were mainly results of the intensive levels rose above SMW (Hohensinner et al., 2004). Since channel incision following river straightening and hydropower many Danube fish species have relatively diverse structureplant construction. Historical and current surveys of the main and niche-specific requirements, successful reproduction channel bottoms showed that this incision measured up to 3.2 and upbringing of juveniles are strongly interrelated with the m between 1812 and 1991. Aggradation of fine sediments accessibility of adequate habitats during different life-history during floods additionally raised the floodplain surface by stages (Schiemer & Waidbacher, 1992; Schiemer et al., 1994; up to 1.5 m depending on the location in the study site. The Schmutz & Jungwirth, 1999; Jungwirth, 1998; Jungwirth et intersection of the modelled terrain surfaces (DTMs) from al., 2000; Zauner & Eberstaller, 2001). In the former Austrian 1812 and 1991 yielded an erosion volume of approx. 20 Machland, when 57 % of the AZ were flooded at BW, 56 % million m3 and sedimentation of 34 million m3. Accordingly, of the AZ offered surface-connected water bodies that were until 1991, 14 million m3 of bed material (gravel, sand, silt) accessible for the fish fauna. This extensive surface area of the remained in the investigated area due to the missing lateral interconnected channel network represented persistent migraerosion in the channelized Danube River. tion corridors to the diverse floodplain habitats at changing flows and supported a broad spectrum of aquatic organisms within the floodplain water bodies (Jungwirth et al., 2002). 5. Discussion Besides active overflow - according to the DTM data - seepage inflow of river water to the floodplain aquifer was apparently also a frequent phenomenon at mean flow 5.1. Characteristics of natural hydrological throughout the study site (modelled assuming a long-lasting connectivity in 1812 stationary mean flow situation). Similar conditions seem to The results point to the large areal significance of frequent have existed even at low flow situations. Seepage inflow, water level fluctuations between LW and SMW (LFP) in particularly in isolated floodplain terrain, contributed to the the former Danube river-floodplain system. The LFP, which wide-ranging expansion of the LFP and the HFP. Each flooaffected larger areas than the HFP, permanently altered aquatic ding process (active overflow, backwater flooding, seepage habitat conditions within the river’s gravel-bed. Therefore, inflow) thereby has specific effects on the riverine ecosystem the LFP-zone offered a broad spectrum of regularly renewed in terms of e.g. thermal conditions, nutrient enrichment and aquatic/semi-aquatic micro- and meso-habitats and hindered migration possibilities (Hughes, 1980; Trémolières et al., © 2007 Lavoisier SAS. Tous droits réservés

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Figure 6: Area shares of the floodplain terrain in the active zone (%) in relation to depths of the groundwater table below the terrain surface (m) in 1812 and 1991. Depths refer to the mean water situation (MW) and are presented as half-meter classes.

1993; Amoros & Bornette, 2002). For example, in the Austrian Danube Floodplain National Park downstream of Vienna, three phases of connectivity between the main channel and the floodplain were identified that characterize the current hydrological conditions within the river-floodplain system (Heiler et al., 1995; Hein et al., 1999a, 1999b; Tockner et al.,1999, 2000a, 2000b): (1) a disconnection phase at low flow when the floodplain water bodies are separated from the main channel, (2) a seepage connection phase at mean flow associated with increasing discharges in the main channel and (3) a surface connection phase due to active overflow from the main channel to the floodplain at higher discharges. During the first phase, the floodplain water bodies are controlled by nutrient-poor groundwater and therefore exhibit lower planktonic primary production. As the Danube river water has high nitrate and inorganic particle concentrations, which increase with discharge, the second phase (depending on water retention time) is characterized by high algal biomass and primary production due to seepage inflow from the main channel. At higher discharges during the surface connection phase, active overflow transports matter and creates migration corridors for aquatic organisms within the floodplain, but primary production decreases. The modelled situation of the Danube River in the Machland in 1812 supports the conclusion that almost 100 % of the AZ even at low flow showed hydrological conditions corresponding to the seepage connection phase observed © 2007 Lavoisier SAS. Tous droits réservés

today in the national park downstream of Vienna. Only remote areas at the floodplain margins were dominated by groundwater inflow from the hillslope aquifers of the hinterland. Increasing discharge up to SMW - according to the modelled DTMs - intensified seepage inflow from the Danube River to the floodplain aquifer. At stages above MW/SMW, active overflow in low-lying vegetated floodplain areas and in abandoned interconnected channels gradually increased. Based on current discharge data, the modelled historical conditions showed that the seepage connection phase predominated over approximately 280 days a year. Throughout the rest of the year, hydrological connectivity was mainly attributed to surface connection (active overflow). The disconnection phase generally played a minor role within the former river-floodplain system and presumably occurred only at extreme low flow and/or far away from the active channel system. 5.2. Human-induced impacts on hydrological connectivity in 1991 In 1991, the strongly hampered hydrological and fluvial dynamics were clearly reflected by drastically reduced former “flow pulse”-affected areas at water level fluctuations between LW and BW. Following river straightening and hydropower plant construction, the floodplain was mostly decoupled hydrologically by dikes from the flow regime of the Danube River. Additionally, two pumping stations, located


at the confluences of two dominating backwater systems to the main channel (Fig. 4e), drained the floodplain in order to lower the groundwater table at river stages between LW and approximately SMW. As seepage inflow of Danube water to the floodplain system was largely blocked by dikes (ca. 0.25 m3 s-1 at LW, 0.50 m3 s-1 at MW, ca. 2 m3 s-1 at BW), the water and nutrient supply mainly depended on sparse seepage inflow from the hillslope aquifer (0.58 m3 s-1 at LW, 0.86 m3 s-1 at MW) and minimal inflow of some smaller brooks (2.55 m3 s-1 at MW; AHP, 1963; Breiner, 1976; unpublished data from the hydrographic services of Lower/Upper Austria, 1999). The artificially lowered groundwater table kept the floodplain system in the status of groundwater-fed disconnection phases over approximately 280 days per year. At stages between SMW and BW, the pumping stations shut down but the weirs remained closed. While connectivity between the Danube River and the floodplain was still blocked by the weirs, hydrological connectivity gradually increased inside the floodplain due to the inflow from the hinterland and the Danube seepage water. Under these conditions, both backwater flooding in interconnected floodplain areas as well as seepage inflow in isolated terrain were the formative hydrological processes (according to DTM analysis; AHP, 1963; Breiner, 1976; Amt der oö. Landesregierung, 1994). When water level rose above BW, the weirs of the pumping stations were opened, thereby re-establishing surface connectivity between the river and the floodplain water bodies. At this stage, active overflow of Danube water played a significant role within the floodplain, comparable to the surface connection phase in 1812. Nevertheless, unhampered overflow of the dikes between the main channel and the floodplain only began at floods with return frequencies of at least two years. 6. Conclusion The modelled connectivity conditions point to the high significance of active overflow and seepage inflow for the Danube river-floodplain ecosystem in the Austrian Machland prior to channelization in 1812. In particular, the long-lasting and large-area Danube seepage inflow during “low flow pulse” (LFP) phases at water levels between LW and SMW - depending on the duration of the water retention in the floodplain - presumably promoted high planktonic primary production due to the nutrient-rich Danube river water. In contrast to the Austrian Danube Floodplain National Park (that is characterized today by the incision of the Danube main channel and artificially increased differences of levels


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between river and floodplain; Schiemer et al., 1999), the disconnection phases at low flow only played a minor role in the Machland. Generally, disconnection phases represent low or medium production phases (Amoros & Bornette, 2002) when the isolated floodplain water bodies are primarily controlled by internal biological processes (e.g. denitrification, biological uptake, trophic interactions; Heiler et al., 1995; Hein et al., 1999a, 1999b; Tockner et al., 2000a, 2000b). In 1812, however, the natural river-floodplain ecosystem in the Machland – based on the reconstructed hydromorphological conditions – represented a mainly hydrologically controlled system with substantial surface and subsurface nutrient inputs that favored high rates of primary production. With increasing water levels between SMW and BW during “high flow pulse” (HFP) phases, the nutrient supply within the floodplain was gradually determined by active overflow of Danube river water. Although the dissolved nutrient content of the Danube river water has drastically increased since the 1970s (Schwaiger, 1995; Kroiß et al., 1997; FEA, 1999), seepage water from river infiltration is nutrient-rich compared with groundwater supply or inflow from the hillslope aquifer (Trémolières et al., 1993; Tockner et al., 1999; Amoros & Bornette, 2002). We therefore assume a high nutrient content of the river water for the situation in 1812 as well. Consequently, depending on the LFP and HFP phases, a high nutrient supply was apparently maintained throughout the year in the former Danube riverfloodplain ecosystem. Today, the disconnection phases in the Machland are artificially prolongated and prevail most of the year. The HFP mainly depends on backwater flooding from small brooks and subsurface inflow from the hinterland; sparse Danube seepage inflow additionally contributes to connectivity in isolated floodplain terrain. Besides the drastic loss of floodplain water bodies since 1812, the human-induced alteration of hydrological connectivity substantially changed the Danube river-floodplain ecosystem in the Machland from a former high potential of primary production to current low or medium productive conditions.

Acknowledgements The authors wish to thank the Austrian Science Fund (FWF) for funding this project (Grant number: P14959).

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