Confluence effects in the Sense River. Eawag - Internship: Christina Riedl

Transcription

1 Confluence effects in the Sense River Eawag - Internship: Christina Riedl

2 Introduction Hydro-morphological mapping of confluence effects at the Sense River

3 Background The Network Dynamics Hypothesis: How Channel Networks Structure Riverine Habitats (Benda et al. 2004) Morphological signs of confluence effects could be: o Sediment deposition o Woody debris o Changing substrate size o Gradient changes o Terraces o Floodplains o Midchannel bars o Channel instability o Side Channels

4 Background The Network Dynamics Hypothesis: How Channel Networks Structure Riverine Habitats (Benda et al. 2004) UPSTREAM Zone of interference: Lower gradient Wider channel Finer substrate DOWNSTREAM Zone of mixing: Higher gradient Narrow channel Deeper pools Larger substrate size

5 Scientific challenge & hypothesis o Do confluence effects exist in the Sense River? o How do tributary confluences modify channel morphology in the Sense River? o Which parameters influence the likelihood of confluence effects? compared to Benda et al The likelihood of confluence effects increases with the ratio of tributary to main stream size. The likelihood of confluence effects decreases with the number of barriers. o Does the problem of fragmentation occur at the Sense River?

6 Methods Field survey Hydro-morphological mapping Measured parameter: o Discharge o Temperature o Channel width o Channel depth o Confluence area o Dams Visually estimated parameter: o Identifiable confluence effects at the confluence area o Identifiable confluence effects upstream and downstream of the confluence o Embeddedness & colmation o Aquatic vegetation o Eco-morphology of the tributary Analysis Descriptive statistics Correlations Logistic regression

7 Results Study area of the Sense river

8 Results Situation of the tributaries at the Sense River.

9 Results Do confluence effects exist in the Sense River? no confluence effects confluence effects numbers percentage 66,2 33,8 100

10 Results How do tributary confluences modify the channel morphology in the Sense River? Woody debris no morphological changes low medium numbers percentage 29,7 4,7 2,0 high 1 0,7 Sediment deposition no morphological changes low medium high numbers percentage 26,4 7,4 2,0 1,4 Changing substrate size no morphological changes finer substrate coarser substrate numbers percentage 5,4 29,7 2,0

11 Results The likelihood of confluence effects increases with the ratio of tributary to main stream size. Cox & Snell R-Quadrat 0,104 Nagelkerke R-Quadrat 0,171

12 Results The likelihood of confluence effects decreases with the number of barriers. no confluence effects confluence effects 100 n= numbers of tributaries no barriers natural barriers artificial barriers

13 Results Does the problem of fragmentation occur at the Sense River?

14 Results Does the problem of fragmentation occur at the Sense River?

15 Conclusion Possible causes for the ineffective confluences in the Sense River: o System characteristics differ from those investigated in Benda et al o River ecosystem too small for measuring geomorphologic confluence effects o Special situation because of the long canyon within the study area o Intermittent streams o High slope of the tributaries o Fragmentation

16 Link to the biotic part Biological functions of tributary confluences: o Increased productivity Wipfli and Gregovich, 2002; Fernandes et al., 2004 o Important spatial refugia for mobile species Scrivener et al., 1994; Kupferberg, 1996; Cairns et al., 2005; Fraser et al., 1995; Nakamoto, 1994 o Unusual, unique ecological opportunities Kreb and Budiono, 2005 o Increased physical heterogeneity Rice et al., 2001b; Kiffney et al.,2006

17 Link to the biotic part o Important compensatory function in regulated, degraded river systems o Tributaries are buffers for nutrients loading, flood Petersen et al., 2001 o Tributaries supply dammed main streams with water and sediments Stevens et al., 1997 Confluences are biotic hotspots, which are important for the general health of riverine ecosystems