POST-DAM GEOMORPHIC TRANSFORMATION OF THE COLORADO RIVER IN ITS DELTA AND IMPLICATIONS FOR CONTROLLED FLOW RELEASES

There are few places on Earth where ecosystem rehabilitation is more daunting than the delta of the Colorado River. The Colorado River has the largest reservoir size in relation to its mean annual flow of any large river in North America; most of its sediment supply is completely blocked in upstream reservoirs; and, stream flow does not enter the estuary in most years. As a result, the true modern delta has shifted downstream ~100 km, and the formerly navigable river is now intermittent or ephemeral in the abandoned portion of the former delta. Delta rehabilitation efforts that include small controlled floods intended to rejuvenate riparian vegetation must consider the new relationship between stream flow and the transformed geomorphology of the delta’s river corridor. The history of post-dam delta channel change is dominated by variable-magnitude floods that punctuate ever-decreasing flows. The river channel in the upstream 35 km of the delta incised 2 to 3 m following early flow releases from Hoover Dam in the 1940s, permanently abandoning surfaces active in the 1930s. A period of relative geomorphic quiescence followed, as upstream water development further reduced delta flows. Subsequent reservoir filling and wet conditions in the 1980s caused the largest post-dam floods in the delta, and resulted in 10s to 100s of meters of lateral migration and ~1 m of additional bed incision. Smaller flow pulses in the 1990s and 2000s further incised the thalweg to its minimum elevation, and the active channel narrowed. Lidar data and satellite imagery show that the degree of bed incision decreases downstream to an area where deposition occurred in the 1980s floods. In 2014, an experimental pulse flow roughly 5% of the typical pre-dam flood peak was released to the river channel near the apex of the pre-dam delta. Suspended-sand concentrations decreased rapidly downstream, in part because infiltration rapidly attenuated the pulse and greatly decreased its volume. Topographic change was confined to sub-meter-scale bed reworking in the relatively narrow channel where post-dam bed incision allowed for larger depths and flow velocities. As a result, post-dam channel change restricted the areal extent inundated by a controlled flow pulse and greatly reduced the area where an active floodplain might be formed and maintained.