ASSESSING EROSION AND RE-DEPOSITION OF RELIC (PRE-DAM) SAND IN MODERN COLORADO RIVER SANDBARS FROM GEOCHEMICAL TRACERS

Sand supply and Colorado River discharge below Glen Canyon Dam, Arizona, USA, are being experimentally managed for purposes of rebuilding and maintaining sandbars in Marble Canyon, the 100 km-long reach of the Colorado River below the Paria River. High-flow experiment (HFE) dam releases are intended to redistribute sediment supplied by the Paria River downstream and build sandbars without eroding existing deposits, particularly from scarce pre-dam eddy, channel-margin, and terrace deposits. Previous sediment budgets estimate that new sand entering Marble Canyon is primarily supplied by the Paria River (more than 83%) whereas pre-dam sand is primarily of Colorado River origin (more than 92%). This marked difference is the basis of our approach using geochemical tracers to determine the relative amounts of Paria-derived sand and pre-dam sand contained in sandbars deposited by HFEs. Fine and very fine sand from the Paria and Colorado Rivers contain abundant quartz and potassium (K) feldspar, and Colorado River sand also contains albite, a sodium (Na) feldspar. Sediment geochemistry shows that, compared to Colorado River sand (n=7), Paria sand near the confluence (n=10) had on average 35% higher aluminum (Al)-normalized K content and 27% lower Al-normalized Na content. Sandbars created by a HFE in November 2012 were sampled in May 2013 as part of an annual monitoring program. The Al-normalized K content of fine and very fine sand from the HFE generally decreased downstream, which suggests an increasing contribution of relic Colorado River sand with distance from the tributary sand source. Al-normalized Na contents were locally elevated in sandbars 35 and 50 km downstream of the Paria, indicating that smaller drainages in Marble Canyon could also influence sediment-geochemical tracers. A broader sampling program was conducted in May 2014 that included washes on the north rim of Marble Canyon and the Paria River 2-6 km upstream of the confluence. By better constraining end member compositions of sand sources to Marble Canyon, it is expected that a clearer picture will emerge about sand mobilization and deposition during HFE.