The impact of fine sediments in small rivers : method development and effects on brown trout redds

Native brown trout populations are declining in Swiss rivers. This could be due, among other reasons, to a clogged riverbed caused by fine sediment deposition, leading to a decrease in interstitial flow and therefore in a reduced oxygen supply to the salmonid embryos. Furthermore, suspended sediment (SS) could directly harm health and fitness of free swimming fish. The aim of this dissertation was to develop and apply methods to measure SS and the effects of weekly fine sediment infiltration and net fine sediment accumulation over the entire egg incubation season on oxygen concentrations in artificial redds and the survival of the implemented brown trout eggs. Furthermore, the effects of riverbed structure, redd morphology and hydrological and hydrogeological conditions on interstitial oxygen and egg survival was assessed. In addition, source areas of SS and organic matter were assessed by C/N atomic ratio, 13Ctot, 13Corg and 15N isotopes. The study was conducted at three sites named A, B and C, from up- to downstream along the canalized and partly stabilized river Enziwigger in the Swiss Plateau. Data were collected weekly or measured continuously during two spawning seasons (2009/10 and 2010/11) from November to March in a total of 36 redds.
Weekly fine sediment infiltration rates in redds were relatively high and generally increased with higher SS concentrations. Both, infiltrated sediments and SS showed strong temporal variations between low flow and peak discharge conditions. Fine sediment infiltration was at maximum during high flow events with sediments mainly in the size of sand (0.063 - 2 mm). These sediments originated for the most part in the upper watershed. Small amounts of fine sediments infiltrated during base flow periods with particles mainly in the size of silt and clay (< 63 µm) and with increasing organic matter concentrations. Organic matter was generally of allochthonous origin and major sediment source areas were pasture and arable land during those low flow periods.
Less fine sediment accumulated over the entire egg incubation period in upwelling zones on the local scale and within areas of higher mean water levels due to corresponding flushing of fine sediments. Even though SS and bedloads increased from up- to downstream, less fine sediment accumulated downstream. Higher flushing of fine sediments and generally increased sediment dynamics downstream due to higher water levels are probably the reasons for this observation. Increased sediment dynamics also caused remarkably scouring of redds: 50% of the redds in the two downstream sites were excavated or buried during high flow events in early winter due to sediment movements. Redd loss at the upstream site A was substantially lower (8%).
The high permeability of the redd substratum and the typical pit-tail structure of redds led to high dissolved oxygen (DO) concentrations in redds shortly after redd construction. Specific infiltration rates q decreased substantially within one month due to riverbed sediment displacements and fine sediment infiltration. This resulted in lower DO concentrations in redds. In individual redds, DO concentration decreased temporally to almost 0%, leading to a depleted redd environment unfavorable for embryo survival. Interstitial DO concentration and q generally increased during high flows. In contrast they decreased during the falling limb of the water level, likely indicating exfiltration of depleted ground- or interstitial water. Similarly, DO concentrations decreased under prolonged base flow conditions. This paralleled the higher percentage of silt and clay particles in the infiltrated sediment, probably triggering riverbed clogging and therefore reducing q.
Even though organic matter in SS increased from up- to downstream due to an increase of pasture and arable land downstream of the river, egg survival was better at the downstream sites. Organic matter concentrations were with means between 5.1% at site A and 6.5% at site C relatively low. The low egg survival at site A was likely due to the high fine sediment accumulation at the site, triggering low specific infiltration rates and consequently decreased DO concentrations. This was especially true at spots with low mean water levels, where flushing of fines is inhibited.
Enhanced soil erosion processes on pasture and arable land are expected with increasing heavy rain events and less snow during winter seasons due to climate change. Consequently, SS and organic matter in the river will increase, which will possibly affect brown trout negatively. Furthermore, a higher frequency of high flows in the future could potentially enhance scouring of redds especially in the downstream sites, which could further reduce egg survival rates.