9708 - Laserfiche WebLink

Home Browse Search

FLUCTUATING FLOWS AND BACKWATER HABITATS 1139 .10% lIll!20% 1!l1l30% 040% 1,0 CD .!: 0.8 t: lI:I ~ 0,6 0:: 5 0.4 ~ .. o g- 0.2 .. a. 0.0 1 2 3 4 5 6 Backwater Figure ] 0, Si mulated proportion of the day's invertebrate production remai ning after water exchange with the mainstem for backwaters 1, 2, 3, 4,5 and 6 at 10, 20,30 and 40% within-day flow variatiou, Note that at 0% within-day flow variation, backwater volume remains constant and hence, no invertebrates are flushed from the backwater DISCUSSION Results from the flow fluctuation simulations suggest that the most important biological effects of varying the magnitude of within-day flow fluctuations on backwaters are likely to be those associated with the availability of invertebrate prey, including (1) minimum wetted area, (2) proportion of the backwater's volume exchanged with the mainstem and, to a lesser degree, (3) backwater temperature. As within-day flow fluctuations increase in magnitude, our model predicts that more of the backwater will be dry at some point in the day and thus unable (or have a reduced ability) to contribute to the production of benthic inveltebrate prey. This effect appears to be most pronounced for smaller, shallower backwaters, and can represent up to a 60% reduction in wetted area relative to that available at the observed base flow. A reduction in shallow water benthic production in response to flow fluctuation was reported by Blinn et at, (1995) in the main channel tail waters of Glen Canyon Dam on the Colorado River, In their study, the magnitude of daily flow fluctuations was strongly and negatively correlated with exposure of shallow water habitat, resulting in the rapid elimination of both algal assemblages and benthic macroinvertebrates in the varial zone, Furthermore, recovery of shallow water production was a protracted process. with the algae used by chironomid larvae for food and cover requiring several months for re-establishment after little more than a week of fluctuating discharge (Blinn et ai., 1995). It is important to note, however, that the Glen Canyon system differs considerably from the mainst.em backwater system we model here (Glen Canyon has rocky subst.rate, few backwaters, algal beds, colder water and a large proportion of the invertebrates live on the veget.ation rather than in the substrate); differences which may affect. the relationship between substrate exposure and inveltebrate production, In addition, our model assumes that, as within-day flow fluctuations increase in magnitude and a greater proportion of the backwater's volume is exchanged with the mainstem, an increasing proportion of the daily production of invertebrate prey is flushed out of the backwater and unavailable for consumption by pikeminnow. The predicted consequences of this effect are more pronounced for smaller, shallower backwaters, with the shallowest backwater having as much as 60% of its volume exchanged with the main stem on a daily basis at the higher level of flow fluctuation. As a consequence of such water exchange (and assuming a flushing factor of 0.25), more than 30% of the day's invertebrat.e production may be flushed out of the backwat.er and into the mainstem. Although substantial, our estimates of invertebrate loss are significantly lower than those reported for higher- velocity habitats; Blinn et at. (1995) observed a 90% reduction in macroinvertebrat.e mass in a tailwater habit.at within 24 h of fluctuations associated with river regulation. Copyright (i:' 2006 John Wiley & Sons, Ltd, River Res, Applic, 22: 1125-1142 (2006) DOl: IO,L002lrra