Developing a Baseline for Flow Releases below a Water Supply Reservoir on an Ungaged Stream

The potential development of a water supply reservoir on a small, weakly perennial stream in Northern Missouri with no historical hydrologic data resulted in a need to develop hydrologic building blocks that could be used to help establish ecological flow releases from the reservoir. Fifteen non-urban northern Missouri gages with drainage areas < 100 square miles were selected to develop baselines for typical base flows and three types of runoff events in this type of system. Base flows were examined by separating the hydrograph into runoff and base flow components.  The monthly median, 10^th, 25^th, and 75^th percentiles of the base flow portion were used to develop potential base flow releases which could vary both seasonally and on the basis of climatic conditions.  The Indicators of Hydrologic Alteration program was used to evaluate the ranges of magnitude, duration, and frequency for small flow pulses, high flow pulses and overbank flows. The results of this effort were used as a starting point for discussions to develop the final flow release regime that would maintain the reservoir’s capability to provide a needed public water supply while also minimizing alterations to ecological and geomorphic processes downstream.