Natural Flow Regimes of the Ozark and Ouachita Mountain Region

Natural flow regimes of the Ozark and Ouachita Mountain region were identified and quantified to provide a baseline for assessing anthropogenic hydrologic alterations.  The natural flow regime represents environmental conditions to which native aquatic and riparian communities are best adapted.  Significant deviations from these conditions would be expected to alter communities by reducing or eliminating species least adapted to altered conditions.  We identified 67 gauged streams in least-disturbed condition, and with adequate flow records, to represent natural flow regimes of the region.  From 171 flow metrics derived from daily flow records, 10 were selected based on principal components analyses to represent ecologically-relevant aspects of flow regimes:  Magnitude of average, high, and low flow, duration of high and low flow, frequency of high and low flow, timing of flow events, and rates of change of flow.  Gaussian mixture model clustering identified seven distinct natural flow regimes:  Stable groundwater, groundwater, flashy groundwater, perennial runoff, flashy runoff, intermittent, and harsh intermittent.  Percentiles were calculated within each flow regime for all flow metrics to quantify natural flow characteristics and variability expected in the absence of anthropogenic disturbance.  Simulated water withdrawal scenarios, following Arkansas’ instream flow requirements, were used to assess degree of alteration expected.