Impacts of In-Stream Flow Requirements Upon Water Temperature in the Central Platte River

Abstract

This report describes the results of a study to determine the impact of various instream flow criteria upon the number of days that water temperature in the central Platte exceeds 1) the State standard of 32°C, and 2) 35°C. The 35°C level is presumed to be a temperature at which the aquatic biota is more severely stressed than at 32°C. The 35°C level was also justified by Dinan (1992) as a temperature that is close to the "Critical Thermal Maximum" for many fish species found in the river basin. The study involves the application of a computational water temperature prediction model, MNSTREM, to the central Platte River. MNSTREM has been shown to accurately predict stream water temperature on an hourly time scale under highly unsteady conditions (Gulliver, 1977; Stefan et aI., 1980), and has been adapted to incorporate all of the important aspects of natural rivers (Sinokrot and Stefan, 1992, 1993, and 1994). Calibration and verification takes place through the comparison with four years of water temperature record at four stations on the Platte River. The model is then used with the four-year weather record to predict the water temperatures that would have occurred with various minimum instream flow criteria applied to the reach. The impact of these in-stream flow criteria on water temperature is then predicted for the four-year period through comparison, of computer simulations.