GRAIN-SIZE LIMITATION OF SAND STORAGE IN THE COLORADO RIVER IN GRAND CANYON NATIONAL PARK

Analysis of sediment-transport data from the pool-drop segment of the Colorado River in Grand Canyon indicates that, before the construction of Glen Canyon Dam, sand accumulated in this river segment when discharges were generally <320 m³/s, and was either conveyed through or eroded from this river segment under higher discharges. As sand accumulated, the bed and suspended sand fined and sand concentrations increased independently of water discharge; as sand eroded, the bed and suspended sand coarsened and sand concentrations decreased. Thus, in the natural river, the amount of sand that could be stored was limited by the physical coupling between changes in sand grain size and transport. The 1963 closure of Glen Canyon Dam cut off ~94% of the sand supply to the Colorado River at the upstream end of Grand Canyon National Park (GCNP), and operations since 1991 have largely eliminated the lower discharges under which sand naturally accumulated. To better understand the sand transport and storage dynamics in this highly altered river, we began continuously monitoring suspended-sand transport (at 15-minute increments) at five gaging stations located such that they segregate the Colorado River in GCNP into five reaches ranging from 42 to 127 km in length. Results from this monitoring network indicate that, except in the two of these long reaches immediately downstream from the key tributaries that supply the vast majority of the sand (i.e., the Paria and Little Colorado rivers), water discharge and antecedent bed-sand grain size together control whether sand accumulates or erodes. In the two reaches proximal to these two key tributaries, sand accumulates only after large floods in these tributaries. In the three reaches distal to these tributaries, sand accumulates only when dam releases are below ~300 m³/s when the antecedent bed-sand grain size is relatively fine (a “threshold” discharge for sand accumulation indistinguishable from that in the pre-dam river). When the antecedent bed sand is relatively coarse, however, sand can accumulate in these three reaches at discharges of up to ~600 m³/s. As sand accumulates, the bed sand fines, and the discharge allowing further sand accumulation ultimately decreases to ~300 m³/s. Thus, as in the pre-dam natural river, bed-sand grain size limits sand storage in the post-dam altered river.