Instream flow condition and summer water temperature in the Central Platte River

Analyses were conducted to isolate and quantify the effect of instream flow condition on summer river temperature in the Central Platte River downstream of the Kingsley Dam Project, Nebraska. Daily maximum and mean river temperatures and diurnal variation were studied. Relationships between water temperatures and river discharge or flow depth were developed using an analytical solution method and a reference parameter for weather. The theoretical relationships were validated against continuous 5-year field measurements at the river. Different temperature-discharge relationships were found to exist under different weather regimes. Statistical analysis was conducted to quantify the effect of river discharge on temperature exceedence of a water quality standard of 32 °C and degree-days of exceedence. A threshold discharge dividing significant and insignificant flow effects on water temperature was determined by a diminishing return approach. Minimum discharge requirements under given meteorological conditions to meet a river temperature standard were derived for the Central Platte River. The results provide information useful in proper planning and design of reservoir operations and streamflow management. The discharge-temperature relationships and the minimum discharge requirements can be incorporated into a habitat assessment model and a reservoir and hydropower plant management program.