Downstream Effects of Diversion Dams on Sediment and Hydraulic Conditions of Rocky Mountain Streams

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 AmericanWest, 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 d84, and it appears to be magnified in streams of less than 3% slope. Copyright # 2010 John Wiley & Sons, Ltd. key words: flow diversion; dam; fine sediment; stream management; hydraulic alteration; field methods; habitat degradation Received 21 September 2009; Revised 26 January 2010; Accepted 2 February 2010