CDOW monitors Blue River flows
Rafters and kayakers weren’t the only ones watching the relatively high flows in the Lower Blue during Labor Day weekend. Colorado Division of Wildlife (CDOW) experts also kept a close eye on the river, as Denver Water increased the flow out of Dillon Reservoir to deliver water downstream.Steamboat-based aquatic biologist Billy Atkinson said he wanted to try and get an idea of how the three-day release affects aquatic habitat, especially in the prolific trout fishery just below the Dillon Dam and on through Silverthorne, where recent habitat improvements have garnered rave reviews from anglers. Atkinson said he also was looking to see whether the increased volume of water would help flush sediments downstream.With drought conditions prevailing in recent years, the Lower Blue has been flowing at unnaturally low levels for extended periods, as Denver Water captures, then diverts significant portions of the river’s water to the Front Range. The habitat improvement projects in Silverthorne – with Phase 2 planned for 2005 – are aimed at making the most of the available water and enhancing the Gold Medal fishery.
But some biologists say that a steady diet of low flows is bound to have long-term consequences for the river’s ecological health. The science can sound complicated. Experts describing a natural hydrograph, a chart of the river’s flows that in the pre-dam era would show a spike during the spring and early summer snowmelt season and then taper off to lower levels during the late summer and fall, spawning season for many trout species.Native species in the Blue and other Rocky Mountain streams evolved over millenia to adapt to such variable flows. Along with seasonal fluctuations, flow changes from year to year can also be ecologically significant, said David Nickum, director of Colorado Trout Unlimited. In some years, conditions may favor a big spawn, when many millions of young fish are able to thrive. In other years, conditions may be better for large adult fish, resulting in a healthy mixture of sizes and age groups.Flow regimes are also important to shaping habitat, Nickum said. Changing flows may change the configuration of the channel, and over-bank flows are needed to maintain streamside riparian habitat – trees and shrubs that provide shade and add organic debris to the river, food for the micro-organisms at the base of the food chain.Extended periods of low flow can result in warmer water and affect other factors, including dissolved oxygen levels, said Jay Skinner, a CDOW biologist who helps coordinate the state’s instream flow program. Fish stressed by such conditions are often more susceptible to disease and other maladies. Fifty cfs in a channel formed by flows of 500 cfs or more means the system is essentially “out of whack,” Skinner said.
But Skinner also pointed out that, while the hydrograph below Dillon Dam has essentially been flat in recent years, downstream tributaries coming out of the Gore Range pump the flows back up during spring runoff, helping to maintain natural conditions. Monitoring
CDOW also plans electroshocking to sample trout populations in the river later this month, both to track whirling disease and to determine if and how the habitat improvements have affected trout populations, said Tom Kroening, the CDOW manager for the Summit County area.Kroening said CDOW’s most recent research shows a high prevalence of whirling disease in the stretch of river north from Silverthorne to the Blue River campground. In response, the agency has stocked more large size rainbow trout. The grown fish are less susceptible to the disease, he explained. The agency is also studying whether to introduce a recently developed strain of rainbows that is more resistant to whirling disease.The fish surveys should help CDOW and town of Silverthorne officials gauge the status of the Gold Medal fishery.
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