CHANNEL COMPLEXITY ON THE DOLORES RIVER: USING PLANET LABS IMAGERY TO MEASURE SIDE CHANNEL LENGTH FOR DIFFERENT DISCHARGES DURING 2019 RELEASE FROM MCPHEE RESERVOIR

The Dolores River drains the western San Juan Mountains in Southwestern Colorado. The construction of McPhee dam in 1983 to hold water for agriculture has greatly impacted the river downstream of the dam due to reduced frequency of high discharge events and encroachment of vegetation into the channel. One of the greatest impacts of the dam is simplification of the riverine environment including loss of side channels which provide important habitat for native fish species (round tail chub, blue head sucker, and flannel mouth sucker). The protection of these fish is crucial to prevent them from being placed on the endangered species list which would place more regulations on the use of water for agriculture as well as threaten the extinction of these native fish species, thus a challenge for water managers in this river system is to sustain habitat for these threatened native fish species while also conserving scarce water for the surrounding agriculture industry. To evaluate the ability of releases from the dam to inundate side channels in the modern river, we analyzed Planet Labs Planetscope imagery collected during a managed release in the summer of 2019 using ArcGIS Pro. At each of 6 discharges from ~80 to ~3500 cfs, we digitized and calculated the length of inundated side channels for 6 study segments.

We find that higher discharges yield an increase in side channel length. Greatest side channel length increases were found in places with minimal valley wall constriction, but the most constricted segments showed little to no sensitivity to discharge. The greatest increase in side channel length occurs at ~2000 cfs, however this increase is partially due to ponds forming adjacent to the main channel, which may not necessarily create habitat, and could even be considered counterproductive if young fish are trapped in said ponds as the water recedes.

This work demonstrates the value of daily satellite imagery for monitoring the impact of controlled releases. Despite its moderate spatial resolution, the Planet Labs imagery allows us to see that discharges surpassing the aforementioned discharge threshold may not inundate enough additional side channels to justify the additional water usage. This finding will help inform future flow release strategies to best satisfy the diverse stakeholders.