Effects of experimental flow regulation on invertebrate drift and stranding in the Flathead and Kootenai Rivers, Montana, USA

Abstract

Studies were conducted to determine the effects of experimental manipulations of discharge on invertebrate drift in two regulated rivers in northwestern Montana, USA. During these studies the discharge regime in the Flathead River was characterized by frequent flow fluctuations, while in the Kootenai River high discharge was maintained for much longer periods before flow was reduced to minimum discharge. The magnitude of the response of invertebrates to disturbance was different in the two rivers, in part because of the different frequencies of flow changes. Midstream invertebrate drift increased an order of magnitude during increasing discharges in the Flathead River but was not substantially increased during decreasing discharges. When the prior discharge regime had been sustained at high levels in the Kootenai River, invertebrate drift densities as high as 300000/100 m 3 were measured along the shoreline following reductions in discharge, both immediately after flow began to decrease and after dark on the same day. There was also more recolonization of shoreline areas and more stranding of insects following dewatering of nearshore regions when there had been sustained high discharge levels prior to the flow reduction. More insect stranding occurred during a faster rate of decrease in discharge (50000 to 100000 organisms m-2).

Causes and consequences of invertebrate drift in running waters: from individuals to populations and trophic fluxes

“…A key consequence of this conceptual model is that the absolute magnitude of hydraulic stress may be less important than its direction and rate of change in determining the response of invertebrate drift to flow variation (Imbert and Perry 2000). Empirical support for this model comes from drift observations during hydropeaking in regulated rivers, where differences in drift concentration between ascending and descending points along a hydrograph are regularly observed (Perry and Perry 1986;Patterson and Smokorowski 2011;Miller and Judson 2014).…”

Effects of streamflow reductions on aquatic macroinvertebrates: linking groundwater withdrawals and assemblage response in southern New Jersey streams, USA

“…Associated reduction in streamflow resulting from changes in surface-and ground-water use greatly affects the suitability of a stream to sustain many types of aquatic fauna (Klein 1979). For example, fluctuations in streamflow and repeated exposure of areas along stream margins have been shown to result in slow recovery and decreased production of aquatic-macroinvertebrate assemblages (Perry andPerry 1986, Blinn et al 1995). Water withdrawals for agricultural, municipal, and other uses have been found to markedly reduce streamflow, resulting in a loss of habitat for stream biota (Postel 2000, McKay andKing 2006).…”

Evaluation of Juvenile Fall Chinook Salmon Stranding on the Hanford Reach of the Columbia River, 1999 Annual Report.

“…Our long-term results corroborate these findings, since midges comprised a higher proportion of the macroinvertebrate community on substrates exposed for longer durations. Perry and Perry (1986) reported slower recovery of biotic communities in lotic ecosystems with daily fluctuations than in systems with less frequent fluctuations. Brusven et al (1974) suggested that prolonged dewatering of the Hell's Canyon reach of the Snake River reduced primary production, causing a lag in recolonization of invertebrates when conditions again became favorable.…”

“…found appreciably higher stranding of invertebrates on dewatered substrate in the fall than spring, mainly because of their smaller size and reduced mobility in the fall. Perry and Perry (1986) found that more insects were stranded during a faster rate of decrease in discharge. Our exposure tests were artificially created, thus dewatering occurred much faster than the hydropeaking-induced changes in discharge and subsequent dewatered substrates.…”

“…Many invertebrates may have become adjusted to the frequent disturbances from hydropeaking discharges. Perry and Perry (1986) suggested that many invertebrates had become adjusted to the frequent changes in discharge due to hydropeaking in the Flathead River downstream of Hungry Horse Reservoir, Montana. Although boat sampled drift densities were considerably higher than from shallower nearshore samples, we found no significant effect of discharge, sample depth, or time of day on invertebrate drift density from boat sampling.…”