In many rivers, summer and fall baseflow periods are particularly critical for survival of fish and other aquatic organisms. Reduction in these baseflows can have severe ecological impacts. In decreasing order of severity, these effect of reduced baseflows can range from completely drying the channel and lowering the water table (desiccating aquatic and hyporheic organisms, and potentially dewatering riparian vegetation), drying shallower parts of the channel while maintaining isolated pools (eliminating connectivity of surface waters, increasing predation by terrestrial animals, and reducing water quality), or, reducing the rate of flow and velocity (causing water temperatures to rise and/or dissolved oxygen levels to drop).
The term instream flows is commonly used to designate the flows released from the reservoir to maintain the ecological health of the river. Because of the severity of the impacts of reduced baseflows, until recently most efforts to establish instream flow releases from dams have focused on setting minimum flows at baseflow. In many US rivers, the PHABSIM model has been widely used as a tool to set instream flows. It is based on curves that relate the preferences of various fish species and life stages to] different habitat characteristics (principally velocity, depth, substrate), and a hydraulic model that relates the habitat characteristics to flow. One problem with the approach is that the existing hydraulic models (at least those practical to apply to this problem) are too crude to model flow characteristics at the scale to which the fish actually respond. Also, the model assumes that the channel dimensions and shape remain constant, an assumption that may not be correct and which requires a historical channel study and monitoring to confirm.
Increasingly, instream flow requirements also include provisions for flushing flows, deliberate high flow releases designed to mimic the effects of natural floods, to remove fine sediment accumulated on the bed (especially in spawning gravels) and in an attempt to maintain pre-dam channel form. Because dams alter both flow regime and sediment load, it is impossible to recreate pre-dam flood conditions. Some flushing flow objectives may be incompatible. Thus, it is essential that the specific ecological objectives of the flushing flow be identified, and these ecological objectives be translated into specific physical changes that can be effected by certain flows.
New Exchequer Dam, Merced River (photo by Kondolf 1994)
SELECTED RELEVANT PUBLICATIONS
Moyle, P.B., and G.M. Kondolf. 2000. Fish bypass flows for coastal watershed: Review of proposed approaches for the California State Water Resources Control Board, June 2000.Kondolf, G.M., E.W. Larsen, and J.G. Williams. 2000. Measuring and modeling the hydraulic environment for assessing instream flows. North American Journal of Fisheries Management 20:1016-1028.
Kondolf, G.M. 1998. Development of flushing flows for channel restoration on Rush Creek, California. Rivers 6(3):183-193.
Wilcock, P.R., G.M. Kondolf, W.V. Matthews, and A.F. Barta. 1996. Specification of sediment maintenance flows for a large gravel-bed river. Water Resources Research 32(9):2911-2921.
Wilcock, P.R., A.F. Barta, C.C. Shea, G.M. Kondolf, W.V. Matthews, and J. Pitlick. 1996. Observations of flow and sediment entrainment on a large gravel-bed river. Water Resources Research 32(9):2897-2909.
Kondolf, G.M., and P.R. Wilcock. 1996. The flushing flow problem: Defining and evaluating objectives. Water Resources Research 32(8):2589-2599.
Castleberry, D.T., J.J. Czech, D.C. Erman, D. Hankin, M. Healey, G.M. Kondolf, M. Mangel, M. Mohr, P.B. Moyle, J. Nielsen, T.P. Speed, and J.G. Williams. 1996. Uncertainty and instream flow standards. Fisheries 21(8):20-21.
Kondolf, G.M., G.F. Cada, and M.J. Sale. 1987. Assessment of flushing flow requirements for brown trout spawning gravels in steep streams. Water Resources Bulletin 23:927-935.
Kondolf, G.M., J.W. Webb, M.J. Sale, and T. Felando. 1987. Basic hydrologic studies for assessing impacts of flow diversions on riparian vegetation: Examples from streams of the eastern Sierra Nevada, California. Environmental Management 11:757-769.
Kondolf, G.M., and M.J. Sale. 1985. Application of historical channel stability analysis (HCSA) to instream flow studies. In F.W. Olsen, R.G. White, and R.H. Hamre (eds.) Proceedings of the Symposium on Small Hydropower and Fisheries (pp. 184-194). American Fisheries Society, Bio-Engineering Section and Western Division, May 1-3, 1985, Denver, Colorado.