SEDIMENT MOBILITY ON THE DOLORES RIVER DURING THE HIGH PEAK RUN-OFF OF 2017

The installation of the McPhee Dam on the Dolores River, CO in 1984 has had a profound influence on downstream channel geometry due to a change in flow regime and sediment supply. The mean peak discharge from spring runoff has decreased by nearly 3000 cfs after the dam installation. This decrease in peak discharge combined with sustained low flows during the dry season has caused channelization and the abandonment of floodplains, resulting in an overgrowth of willow along the stream channel and bank stabilization. River channelization and low sediment flushing as a result of low flows have dramatically reduced habitats for the native warm water fish populations whose numbers have been declining dramatically since the installation of the dam.

The unusually large 2016-2017 snowpack produced a high spring run-off, allowing water managers to release above 1000 cfs for over six weeks including four days at 4000 cfs. In order to test the hypothesis that this rare high flow release will modify the river channel geometry by scouring fine sediment and reworking and eroding floodplains, we collected pre-spill geomorphic data from a series of three cross sections and Structure from Motion data collected at ~10 cm resolution by pre- and post-spill drone flights along our research site near Slick Rock, CO.

Initial data from the re-surveyed cross sections suggest that although the channel and adjacent floodplain were inundated by the flow there was only minor reworking of sediment. Reworked sandbars in the willow dominated floodplain indicate that this environment did experience high flow velocities, yet the willows appeared to be undisturbed. High flows were able rework a mid-channel bar, yet scouring of the downstream pools and riffles was minimal. Once captured, Structure from Motion data will be used to gain an understanding of geomorphic changes to floodplain surfaces not captured by our three cross-sections. Overall, there appears little effect of this rare hydrologic event on the post-dam channel geometry of the Dolores River, suggesting that water managers have little power to reverse the effects of flow regulation on this semi-arid mountain river.