Stocked fish sustain gold medal status through Silverthorne
By Bob Berwyn
SUMMIT COUNTY — Even though it’s classified as a gold medal trout stream, the Blue River below Dillon Reservoir is not all it could be. State water quality experts and biologists will scrutinize the river the next few years after recent sampling showed that there aren’t nearly as many aquatic insects in the riverbed as needed to sustain a healthy aquatic ecosystem. The summary of the assessment is online here.
The findings didn’t surprise Jon Ewert, an aquatic biologist with Colorado Parks and Wildlife who has been sampling and studying the Blue (along with other streams in the region) for several years.
“We’ve known for years that there’s a lack of productivity in the Blue,” Ewert said, describing how repeated sampling of trout shows very slow growth rates and fish in poor body condition — consistent with similar sampling results going back to the 1980s.
The gold medal fishery is essentially sustained by restocking the river with big hatchery fish near the end of their natural life cycle, he explained. Fish sampling showed problems far downstream, not just in the reach directly below the dam, he added.
The sampling results prompted the Colorado Water Quality Control Division to propose listing the Blue River as impaired under a relatively new rule that sets thresholds for aquatic life use.
Similarly, Gore Creek through the town of Vail — another gold medal segment — is also proposed for listing as impaired, triggering a 10-year window to develop and implement a cleanup plan.
But after input from local water quality experts, the division will instead designate the Blue River for monitoring and evaluation. That means the state agency will work with other stakeholders to collect more data, trying to establish why there is a dearth of bugs in the popular angling stream.
Water experts from Summit County and other jurisdictions challenged the initial move to list the Blue River and other stream segments as impaired, claiming that the state-set thresholds — adopted after 10 years of study — may not be applicable in rivers below reservoirs.
For example, Aurora officials questioned whether or not the data collected below a dam should be evaluated as being representative of an entire stream segment. They suggested that changes in natural temperature alterations, low dissolved oxygen, sediment, nutrient composition and hydraulic modifications may alter the biological community below reservoirs.
In Summit County, the Northwest Colorado Council of Governments (NWCCOG) opposed the provisional aquatic life listing for the portion of the Blue River below Dillon Reservoir due to an “alteration” of the expected condition resulting from low water temperatures, lack of leaf detritus, and alterations in sediment carrying capacity.
NWCCOG’s water quality program managers proposed to include this segment on the monitoring and evaluation list in order to work with the WQCD and other interested parties to assess this “altered condition.”More research could help determine whether there is a more appropriate biological threshold for reservoir tailwaters, or to develop a more suitable aquatic life-use threshold for these segments.
There’s plenty of solid science showing that man-made water impoundments may eliminate or reduce local aquatic insect populations through dewatering, reductions in time and duration of flushing flows, and physical river bed disturbance. Impoundments also change temperatures and flow regimes, which are considered to be the most important factors in determining the composition of the aquatic ecosystem.
As a result, the WQCD agreed that a monitoring and evaluation status may be more appropriate for some segments — but not as an “offramp” for aquatic life listings.
Aquatic insects in a river are the basis of the food chain and their abundance and diversity of a fundamental indicators of ecosystem health in a river. A lack of bugs is a clear warning sign that the aquatic system is not functioning as it should be. The WQCD said it would welcome science-based alternative thresholds to establish “acceptable alternative biological indices and set expectations for the spatial extent of such a threshold.”