Assessing the Ecological Integrity of the Walker River:  How “Natural” is the Walker River Hydrograph
Presented at the 2009 International Symposium on Terminal Lakes

M. K. Liquori, Sound Watershed Consulting and J. Prancevic, CALTECH

The Walker River has served as a major water source for farmers within the Walker River Basin for nearly 150 years.  Between 1958-2007, 55%-131% of natural inflows have been diverted from the entire river for irrigation and other anthropogenic uses, while the remainder of the water is delivered to Walker Lake or lost to evapotranspiration and groundwater.  These diversions and other land-use practices have altered the river geomorphology, however the ecological effects are less clear.  This study examines whether the river has retained its functional ecological integrity, despite these diversions, based on 11 specific alteration types for 5 key factors describing the Natural Flow Regime as postulated by Poff et al. (1997): timing, rate of change, frequency, magnitude, and duration.  In terms of these factors, we have compared the measured flow regime and an approximation of the natural flow regime (each corrected for diversions, storage, and losses) in several reaches of the upper river watershed for the years 2001 to 2007.  As a whole, 78% of the measures were classified as low or no observed alteration.  Generally, the timing of flows have been affected minimally by the diversions at each observed reach.  Hydrograph peaks occur at the same time in both regimes and baseflows are consistent with the natural variation in low flows.  Low flow durations are typically augmented slightly in the current flow regime, but usually by less than two weeks.  For most reaches the snowmelt peak duration was shortened by only a few days, and other peaks (e.g. rain-on-snow) where minimally effected. In all years the rate of change of discharge with respect to time was consistent with rates observed in the natural flow regime on the annual scale, and minimal variations were seen on the storm scale in 2003 (dry) and 2005 (wet).  The temporal variation in hydrograph peaks was highly consistent with the natural regime in all years.  Discharges during low flow periods saw slight stabilization on the scale of a day or two.  The duration of peak flows and baseflows are both highly consistent.  The most significant impacts include 1) a reduction in magnitude of the peak flows at most reaches, with the scale of reduction increasing downstream, 2) the virtual loss of the snowmelt peak at Wabuska during drought years, which resulted in 3 fewer floods, and 3) modest reductions in flow during the months immediately preceding the snowmelt peak in a few reaches during some years.  A significant portion of the effects at Wabuska appear to be influenced by substantial losses to groundwater in Mason Valley.  Overall, the river appears to have retained much of the integrity of the natural flow regime according to these factors.
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