DECADAL-SCALE SIMPLIFICATION OF THE DOLORES RIVER, SW COLORADO, IN RESPONSE TO FLOW REGULATION

The Dolores river is a snowmelt-fed mountain river that drains from the western San Juan Mountains into the deserts of the eastern Colorado Plateau. Its waters are central to agricultural activity in southwestern Colorado and nearby tribal lands. Construction of McPhee dam in 1983 drastically altered the flow regime downstream by limiting high flows and raising baseflows. In most years, there is no release of water beyond a ~50 cfs 'fish pool'. The primary response of the river corridor is that the channel has narrowed, simplified, and has become host to encroaching riparian vegetation. A central challenge to water managers is how to best use available water to revitalize the river below the dam and enhance habitat for native fish while meeting the needs of other stakeholders.

Here we present an effort to document and quantify changes to the river from 1983 to present using a variety of imagery sources. Ongoing work includes tracking changes in channel width, side channel length and density, unvegetated bar density, and percent vegetation cover. We are also using Planet Labs imagery to map inundation at different discharge levels of a controlled release in 2019.

Preliminary findings from the historical imagery analysis include: 1) Sections of the channel below the largest tributary (Disappointment Creek) are most dramatically affected by post-dam channel narrowing and simplification. 2) Farther downstream, below the confluence with the San Miguel River, river dynamism and complexity has been better maintained. 3) In the most affected reaches, it now requires nearly twice the discharge to inundate side channels than prior to dam construction.

Preliminary findings from the analysis of the 2019 release include: 1) Canyon-constricted segments of the river are not made more complex with increasing discharge. 2) In the other study segments, side channel habitat is only marginally improved with increasing discharge until discharge exceeds ~2200 cfs. Beyond that threshold, additional increases in discharge do not seem to add to available side channel habitat in meaningful ways.

Our results are clarifying the trajectory and pace of change in the Dolores River corridor while probing the limits of what contemporary release strategies can do to support native fish habitat.