A whitewater release from McPhee Dam this month will provide boating fun on the Dolores River, but it also will have ecological benefits.
Aquatic biologist Jim White with Colorado Parks and Wildlife spoke at a community meeting in Dolores about planned fish surveys, population data and survey techniques.
Parks and Wildlife works with McPhee Reservoir managers to manage downstream flows for three native species that reside in the Lower Dolores River – the roundtail chub, bluehead sucker and flannelmouth sucker. The first several miles below the dam to Bradfield Bridge are managed as a cold-water fishery for brown and rainbow trout.
“Roundtail populations have been good,” White said, “and bluehead and flannelmouth are not as abundant.”
The reservoir holds a 33,500 acre-foot reserve for the native fish needs. The “fish pool” is released gradually throughout the year based on biologists’ input. In the winter, flows below the dam are 20 to 30 cubic feet per second. During summer, they reach 60 to 80 cfs, if there is no whitewater release.
During low water years, the fish pool and farmers share in shortages. When there is a recreation dam release like this year, it is not counted against the fish pool, and the higher flows are managed for ecological benefits, such as channel scouring, timing to benefit the fish spawn, and flood plain sedimentation that replenishes nutrient-rich sediment on the banks for new seedlings.
The number of fish captured during surveys is tied to whether there is a dam release. For example, in 2018, there was no release, and 110 fish were captured for study at the Dove Creek Pump Station. In 2017, during a large release, 160 fish were captured there. Fish counts and surveys are done each year at Slick Rock Canyon, Dove Creek Pump Station, Pyramid Mountain and below the San Miguel confluence.
White explained how a “pit-tag array” installed in 2013 to monitor native fish on the Lower Dolores River works. It is just upstream from the Disappointment Creek confluence.
Native fish captured throughout the Lower Dolores are inserted with an electronic tag, and when they move past the “array” wire above the river, the movement and fish identification is recorded.
So far, 1,421 fish have been tagged. Of those, 38% were flannelmouth suckers, 35% were roundtail chubs and 23% were bluehead suckers. Four percent were smallmouth bass, a non-native species biologists are trying to get rid of because they prey on young native fish.
Since being installed, 157 tagged fish have been recorded passing under the pit-tag array. In 2018, 14 fish were detected, including eight flannelmouth that arrived after April 8. Five of the flannelmouth were tagged in Slick Rock Canyon, two in the Pyramid Mountain Reach and one tagged in 2014 in the San Miguel River.
The first native fish of 2019 passed under the array April 5. They were last detected Oct. 18. On April 16, two flannelmouth were recorded.
‘Suppress the spawn’ to preserve the eggsThe native fish are preparing to spawn in spring, and reservoir releases are timed to support optimum conditions.
For example, when river temperatures below the dam rise to between 43 degrees and 75 degrees, different native fish species spawn. But when that happens before a large volume release from the dam, the sudden rush of water can wipe out the eggs and young fish.
To solve the problem, temperature gauges in the river are monitored. When they approach spawning levels before a planned high-volume whitewater release, managers will release 100 and 150 cfs before and after May 15 to “suppress the spawn,” which lowers the water temperature, thereby delaying the spawn until after the higher flows have dissipated.
Fort Lewis College studies floodplainCynthia Dott, a floodplain ecologist and professor from Fort Lewis College, explained the benefits of the large dam releases on the riparian forests of the Lower Dolores River.
“Floodplain forests rely on high spring flows to scour banks and open them up for new seedlings,” she said.
High flows also deposit nutrient-rich sediment onto the riverbanks and recharge the water table. They also scour depressions in the river bottom that native fish need in low-water conditions.
FLC students have been studying the floodplain condition on the Lower Dolores. They have been measuring the depth of the groundwater and how it rises and lowers in tandem with river flows.
Healthy plant and tree habitat depends on sufficient water table levels. For example, willow density on the riverbanks increased dramatically after the big spill in 2017. In contrast, a recent dry spell without high-volume dam releases resulted in a die-off of cottonwood trees relied on by wildlife and recreationists. A stressed cottonwood will drop a lot of branches.