Flow Sediment Interactions in Managed Rivers: Influence on Ecosystem Structure and Function

Abstract

Flow and sediment regimes are two of the main abiotic factors driving riverine ecosystems, interacting at different temporal and spatial scales. These interactions have a strong influence on habitat composition, ecosystem processes and community composition. Nowadays, flow and sediment regimes are highly altered due to anthropogenic modifications of rivers and floodplains, such as dams or river channelization, which disrupt the main properties of both regimes. The primary goal of this thesis was to understand the role of flow and sediment interactions on ecosystem structure and function in managed rivers and evaluate different techniques that mitigate the impact of rivers regulation. Hyporheic sediment respiration, periphyton biomass and chlorophyll-a were used as indicators of ecosystem function, whereas macroinvertebrates assemblages were used as a structural indicator. The first chapter studied the variability of different indicators of ecosystem structure and function under different flow and geomorphology conditions. The study took place in a section of the river Thur, where a widening restoration was implemented by removing 1 km of lateral protections. This widening section characterized by newly established habitats such as gravel bars is fringed upstream and downstream by channelized sections. Channelized sites showed higher rates of sediment respiration, periphyton and macroinvertebrates compared to restored sites. Restored sites showed a greater temporal and spatial variability in the measured indicators due to the influence of flow disturbance, which was lower in channelized sites. Overall, sediment respiration and macroinvertebrate richness were related to flow variability and geomorphology, whereas periphyton and macroinvertebrate density was influenced mainly by flow variability. The second chapter investigated the influence of flow regulation (residual flows and hydropeaking) on aquatic life stages that can persist in sediments during dry phases (i.e. macroinvertebrate seedbanks). Gravel bars affected by hydropeaking showed greater densities of organisms due to the high frequency of inundations, which increased the drift of animals onto gravel bars. The opposite trend was found in residual flow rivers, where there is a lack of flood occurrence. The third chapter assessed the ecological impact of a Sediment Bypass Tunnel, a structure that routes upstream sediment input around dams, reducing the accumulation of sediments in the dam. Operations of the tunnel create a general decrease in measured indicators (sediment respiration, periphyton and macroinvertebrates), the decrease being related to the magnitude of flow and sediment released at the tunnel. The fourth chapter described the interactions of flow and sediment inputs in two rivers with contrasting management programs (experimental floods and sediment bypass tunnel). Sediment lateral inputs caused local interactions with flow, generating morphological and biotic heterogeneity in the streambed, whereas upstream sediment inputs together with high flows by sediment bypass tunnel created a general decrease in biotic indicators in the system, reducing spatial variability. The results from this thesis showed that each of the measured ecosystem properties is influenced in a specific way by flow-sediment interactions. The results indicated that different techniques can be used to mitigate the negative consequences of flow and sediment regime alterations on ecosystem functioning and structure, highlighting the important role of integrative objectives in river management.