STREAMFLOW AND SEDIMENT MANAGEMENT FOR MAINTENANCE OF RIVER ECOSYSTEMS: NORTH FORK CACHE LA POUDRE RIVER, COLORADO

Streamflow on the North Fork Cache la Poudre River, a tributary of the South Platte River in northcentral Colorado, has been modified by impoundments for a century. Flow management has accommodated agricultural needs within northeastern Colorado. Within the last ten years, however, agreements between stakeholders have led to increased winter flows and stepped-spring and fall releases from Halligan Reservoir that are more suitable for downstream habitat maintenance. Sediment management over the past century has involved periodic releases of sediment during reservoir drawdown, sometimes resulting in catastrophic aggradation within the downstream channel. Trapping efficiency within Halligan Reservoir is 99 percent, resulting in a large delta and transport of fines via fetch toward the dam, with a maximum sediment thickness of 4 m. A bathymetric survey and comparisons to design maps indicate reservoir capacity loss is only 5 percent over the past 62 years. Historical reconstructions of flow indicate that average annual peak flow has decreased by 6 percent, as a result of reservoir operations, with a more significant reduction in baseflows. Reduction in flood-related disturbances has resulted in significant shifts in the population structure of the dominant woody species present along the North Fork, including reduction in disturbance-tolerant species and overall terrestrialization of downstream reaches. Field evidence suggests that the 25-year flood is sufficient to uproot common riparian shrubs and to saturate floodplain soils. Managed 25-year floods may serve as a target flow for generating canopy gaps, creating regenerative habitat, enhancing biogeochemical processes, and maintaining habitat heterogeneity. Recent cooperation between the Cities of Fort Collins and Greeley, current owners of Halligan Dam and Milton Seaman Dam, respectively, heralds the onset of new water management with shared vision planning by stakeholders. Both dams are currently undergoing permitting for enlargement to augment municipal water supplies for the rapidly growing population of the Colorado Front Range. Flow enhancements are being developed for the North Fork River to meet priority conservation targets for riparian vegetation and aquatic organisms.