Downstream effects of diversion dams on sediment and hydraulic conditions of Rocky Mountain streams

Abstract

Reduced streamflow via flow diversion has the potential to limit the sediment-transport capacity of downstream channels and lead to accumulation of fine sediments and habitat degradation. To investigate, we examined the effects of variable levels of flow diversion on fine-sediment deposition, hydraulic conditions and geomorphic alteration. Our study consisted of a detailed field analysis pairing reaches above and below diversion dams on 13 mountain streams in north-central Colorado and southern Wyoming USA. Diversions are ubiquitous across the American West, yet previous comparative studies on the effects of flow diversion have yielded mixed results. Through application of strict site-selection criteria, multiple fine-sediment measures, and an intensive sampling scheme, this study found that channels downstream of diversions contained significantly more fine sediment and slow-flowing habitat as compared to upstream control reaches. Susceptibility to fine-sediment accumulation was associated with decreasing basin size, decreasing bankfull depth and smaller d 84 , and it appears to be magnified in streams of less than 3% slope.

Run-of-River hydropower and salmonids: potential effects and perspective on future research

“…Comparisons between bypassed and upstream reaches of lowhead RoR dams found higher levels of fine sediment and significantly slower water velocity in 13 bypassed reaches as well as significant differences between upstream and bypassed reaches in 32 of 41 hydraulic variables (Baker et al 2011). Based on their modelling results, Baker et al (2011) also concluded that small, low-gradient streams with smaller-sized substrate were more susceptible to fine-sediment accumulation than large streams. However, RoR dams that are regularly overtopped at high flows are expected to experience fewer discontinuities in the morphological and sediment size distribution of stream channels (Kondolf 1997;Csiki and Rhoads 2013;Kibler and Tullos 2013).…”

“…Although evidence in the peer-reviewed literature specific to RoR hydropower is more limited (n = 31), it suggests that water quality, habitat quantity, and geomorphology can also be affected by RoR hydropower operations (Kubečka et al 1997;Baker et al 2011;Nislow and Armstrong 2012;Bilotta et al 2016). Changes to NFR following diversion of flow can affect water quality mainly through changes to temperature regimes, pH, and dissolved oxygen (Valero 2012).…”

“…Changes to NFR also alter channel hydraulic, sediment transport, and geomorphology downstream of low-head dams. Such changes to water quality and physical habitat in turn diminish habitat quality, quantity, and diversity for fishes (Baker et al 2011;Fuller et al 2016). The diversion of flow for RoR hydropower, and subsequent changes to water quality and physical habitats in bypassed reaches, therefore constitutes the first pathway of effect by which RoR hydropower has the potential to influence salmonids.…”

“…The second mechanism by which a reduction in flow from RoR hydropower operations could impact fish is through a reduction in habitat quantity and diversity (Anderson et al 2006;Baker et al 2011;Mueller et al 2011). Overall, the literature demonstrates that the diversion of flow in streams reduces the depth and velocity of water in bypassed reaches, which generally leads to a decline in habitat quantity and quality and, in turn, to a decline in fish biomass and density.…”

“…Coarse woody debris is a key component of fish habitat, as it promotes the formation of pools, limits erosion, and provides cover and refugia (Sedell et al 1990;Mossop and Bradford 2004). Comparisons between bypassed and upstream reaches of lowhead RoR dams found higher levels of fine sediment and significantly slower water velocity in 13 bypassed reaches as well as significant differences between upstream and bypassed reaches in 32 of 41 hydraulic variables (Baker et al 2011). Based on their modelling results, Baker et al (2011) also concluded that small, low-gradient streams with smaller-sized substrate were more susceptible to fine-sediment accumulation than large streams.…”