Removal of obsolete low-head dams is accelerating as an aquatic ecosystem management practice. Dam removal is much less common in the arid and semi-arid West than in the water-rich Midwest, Eastern Seaboard, and Pacific Northwest regions. This paucity of examples makes it important to, when feasible, accumulate case studies of how rivers and aquatic ecosystems in arid and semi-arid regions respond to removal. In 2023, the U.S. Forest Service and collaborators removed the Lake George Diversion Dam on the South Platte River in Colorado. Sediment was excavated from the former impoundment and riparian restoration was undertaken to improve habitat and recreation opportunities. We monitored the benthic macroinvertebrate community as a bioindicator of aquatic ecosystem health using a before-after-control-impact (BACI) design prior to the dam removal and for two subsequent years. Samples were collected and analyzed at impact reaches immediately upstream and below the dam site as well as in an upstream control reach.
Before the dam removal and accompanying restoration, the impact reaches were populated by macroinvertebrate communities characteristic of lentic and degraded habitats and a multi-metric index classified most sites as “Impaired”. This was particularly true in the impoundment (upstream of the dam), which was filled with fine sediment that likely degraded macroinvertebrate habitat and feeding opportunities. Sensitive and intolerant taxa became far more prevalent in impact reaches following the restoration and index values increased by 44% with most sites achieving “Unimpaired” status. Overall density and richness significantly increased at the impact reaches relative to the control reaches and approached reference conditions within two years of the dam removal. Non-metric multidimensional scaling analysis demonstrates significant shifts in the community makeup of both impact reaches towards that of the control reach over time. Our results agree with other recent work from wetter landscapes suggesting that aquatic ecosystems in dammed river reaches can undergo reasonably rapid (two years in our study) recovery from severely degraded conditions to a state more similar to free-flowing reaches. Monitoring will continue through 2026 to assess longer-term trends in recovery following dam removal.