Spatial and Temporal Analysis of Stream Restoration Efforts in Depleted Aquifer Systems

ABSTRACT SPATIAL AND TEMPORAL ANALYSIS OF STREAM RESTORATION EFFORTS IN DEPLETED AQUIFER SYSTEMS by Jeffrey C. Davids Master of Science in Geosciences Hydrology/Hydrogeology Option California State University, Chico Spring 2011 Recent declines in anadromous fish populations have led to several proposals for improving stream health in relation to fisheries. Many of the streams in consideration are impacted by a variety of intensive water uses including municipal, industrial, and agricultural. Consequently, in many cases, the streams have been rendered artificially ephemeral due to surface-water diversions and groundwater pumping. Decades of these practices has caused a lowering of the underlying groundwater table and a subsequent one-way connection between the stream and the aquifer. While extensive study has been conducted on how pumping impacts a stream (via stream depletion factors and analogous approaches), relatively little has been done to analyze how these systems can be restored to benefit fisheries. Specifically, how effective is it to reintroduce water into these depleted streams? To help answer this question, MODFLOW-2005 with the StreamFlow Routine Package (SFR2) was used to examine the (1) time scales, (2) spatial extents, and (3) magnitudes of fishery benefits obtained by reintroducing water into the streams of these historically depleted systems. Furthermore, what are the sensitivities of the timing, spatial extent, and magnitude of these benefits to changes in initial conditions, aquifer geometry, riparian vegetation, and aquifer and stream properties? Three (3) distinct flow Regimes emerged during the course of the parametric modeling investigation. First, Regime 1 occurs when the stream is discontinuous (i.e. dry stream reaches) for both the initial and steady states. Second, a stream that is initially discontinuous at the initial state that regains continuity as a new steady state is approached is classified as a Regime 2. Lastly, a Regime 3 system is one were the stream is fully continuous at both the initial and steady states. In general, the spatial extent of fishery benefits increases as the system moves from Regime 1 to Regime 2 to Regime 3. Restoration benefits in a Regime 1 system will always have a limited, ‘discontinuous’ extent. Regime 2 has the most transient spatial extent, ranging from a discontinuous stream at the initial state to fully continuous stream at steady state. It should be pointed out, however, that fully continuous is not synonymous with fully recovered since the quantity and quality of the continuity are extremely important as well. Finally, restoration in a Regime 3 system has the immediate effect of full stream continuity, or maximum spatial extent. The timescales of fishery benefits observed during the course of this investigation range from the tens into the thousands of years and are a function of a myriad of variables. Some of the most significantly influential timescale variables considered include the size of the basin (DEL), the initial depletion, specific yield (Sy), stream geometry, vertical streambed conductivity (Ksb), horizontal aquifer conductivity (Kx) and streamflow (Q). Full efficacy of stream restoration efforts are not fully realized until the dynamic, two-way connection between the stream and the aquifer has been reestablished. Stream restoration is therefore resolutely linked to aquifer restoration. Further, successful stream restoration is a function of proper, and integrated, management of both surface water and groundwater systems. The timescales involved for full recovery can be quite extensive if not indefinite. A new steady state in the groundwater system cannot be reached if the pumping rate is in excess of the ‘Potential Capture Threshold’. The time it takes for a system to reach a new steady state becomes exponentially longer as the rate of pumping approaches the ‘Potential Capture Threshold’. However, pumping at the ‘Potential Capture Threshold’ can still have adverse effects on the stream. Therefore, management decisions should be based upon the ‘Sustainable Capture Threshold’ to protect both the stream and the aquifer in efforts to avoid unacceptable environmental, economic or social consequences.