Dam induced stranding of trout fry on the middle Green River

Brett Baker and Karrigan S. Börk

While walking along the riverbank after breakfast on the morning of June 14 approximately RM 280, I noticed several desiccated trout fry lying in a depression in the mud. Upon further examination of the surrounding area I located several more dead fry ranging from 24-37mm. Quite puzzled I returned to camp seeking the opinion of my fellow students and Dr. Peter Moyle. After some clarification we collectively recalled that flows had risen steadily following our arrival at the campsite from the 800 cfs level we had observed when putting in at the base of Flaming Gorge Dam. At some point during the night flows reached a peak of 2000 cfs, covering much of the shoreline in water before returning again to the low base flow. Much of the inundated shoreline contained vegetation and rock, which in addition to being refuge from the current could have provided additional protection from the predation of larger trout. We suspect that these fish were stranded when
water levels receded.

Given that nine trout fry were found over a 10 M stretch in Upper Brown’s Park. Based on our data, trout fry extend from the dam to the confluence with the Yampa, a distance of roughly 65 miles or 104,000 meters. Extrapolating from this 10 M stretch gives a rough estimate of daily trout fry standings of 93,600. This number is probably quite high for a variety of reasons. First, many riparian habitats may not be as likely to facilitate stranding, as areas which have scattered depressions (footprints, etc) that can trap water and strand trout fry. 


Second, the rate of water rise and fall associated with daily changes in water released from the dam is determined by distance from the dam. The dam releases are brought up to 2000 cfs for a period of time and then are returned to 800 cfs, but as one moves farther from the dam the rate of increase slows and the plateau is gradually eliminated. The change in water level is gradually normalized as it passes downstream, due to interactions with the stream bed and riparian areas. Thus areas farther downstream may not be as prone to stranding, if the high water is present for a shorter duration and the fry have less time to move into the shallows.

Even if the mathematical exercise above is off by two orders of magnitude, the repeated stranding of trout fry day after day could be a controlling factor in the number of trout surviving to adulthood. Prior investigation of daily variation in stream flow on the Green River have focused on trout condition and habitat use by adults, but additional analysis is needed to determine the effect of the variation on trout recruitment.