RIVER CHANNEL EVOLUTION IN SEDIMENT SURPLUS ENVIRONMENTS: A CASE STUDY OF COLORADO RIVER REACHES IN INCISED RESERVOIR SEDIMENTS

Large dam, and associated reservoir, construction peaked in North America in the mid 20^th century. Most large reservoirs have operated since that time at or near optimal operation levels until the last few decades. Increased water demands and prolonged droughts have led several reservoirs, especially in the Southwest U.S., to lower pool levels not seen since dam closure. Lake Mead and Lake Powell on the Colorado River are two prominent examples of dramatically lowered reservoirs. The river now freely flows through tens of kilometers of former reservoir sediments in both lakebeds. The new river reaches are defined by 30 to 60 m high sand-silt banks, unstable and continually adjusting channel geometry, and active knickpoints at points where the channel has migrated from historical paths to locations where it now encounters buried bedrock ridgelines. The most dramatic knickpoint is the Pearce Ferry Rapid, a drop of ~10 m that has created a perched channel base level for approximately 40 river km upstream. This study presents preliminary analyses of lateral channel movement, knickpoint migration velocity, and baseline morphology and grain size data for these novel and new river segments. Further study is needed to determine impacts on the ecosystem, navigation, and to provide a geomorphic framework for these anthropogenic river channels.