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This study is a detailed investigation of the dynamic interaction between a stream and an alluvial aquifer at Swanton Pacific Ranch in the Scotts Creek watershed in Santa Cruz County, California. The aquifer is an important source of groundwater for cropland irrigation and for aquatic ecosystem support. The potential for groundwater pumping to deplete Scotts Creek stream flows is a source of serious concern for land managers, fisheries biologists, and regulatory agencies due to the presence of federally protected steelhead trout (Oncorhynchus mykiss) and coho salmon (Oncorhynchus kisutch). An understanding of the interaction between the stream and pumped aquifer will allow for assessment of the impacts of groundwater extraction on stream flows and is essential to establishing minimum instream flow requirements. This will aid in the development of sustainable groundwater pumping practices that meet agricultural and ecological needs. Based on findings from studies conducted in similar hydrogeologic settings and Scotts Creek stream survey reports filed by the California Department of Fish and Wildlife, we hypothesized that the stream is directly connected to the aquifer and that abstraction of groundwater from agricultural wells has a measurable impact on Scotts Creek stream flows, particularly during the summer low-flow period. We tested this hypothesis by employing a multi-scale approach combining multiple measurement techniques. Results of field investigations, including extensive direct push subsurface sampling, constant rate pumping tests, dye tracer tests, electrical resistivity interrogation of the subsurface, and long-term passive monitoring of aquifer hydraulic heads and stream stage, are reported. Additionally, results of laboratory falling-head permeameter tests and particle size analyses of aquifer sediments, and numerical groundwater flow modeling (MODFLOW) are presented. Findings indicate that the permeable subsurface formation tapped by irrigation wells is a leaky semi-confined aquifer, overlain by a thin and laterally discontinuous very low-permeability aquitard of silt and clay above which lies Scotts Creek. The aquitard reduces the hydraulic connection between the stream and pumped aquifer resulting in a low and steady streamflow depletion rate over the short term. These results are particularly useful to land managers responsible for sustainable groundwater abstraction from wells that tap into the aquifer. Calculations of stream depletion rate based on aquifer hydraulic parameters and well pumping rates are included to allow land managers to conveniently modify groundwater abstraction practices, minimizing concerns of streamflow depletion. The conclusions presented herein are based on the results of a two year study. Stresses imposed by changes in climate and water resource needs should be examined with rigorous science to determine appropriate management strategies. Additional research, including improvement of the numerical groundwater flow model's representation of the natural system, supplementary subsurface investigations, and continued long-term measurement of groundwater levels, is needed to further quantify the degree of stream-aquifer connectivity and to ensure continued sustainable groundwater management.

The classical theory of estuarine dynamics has been challenged by a number of recent studies regarding the important contributions of tidal, lateral, and Coriolis effects to the subtidal dynamics at leading order. Although the classical theory excluded these processes, Hansen and Rattray’ ( R65) b subtidal circulation. However, the success of the predictive skill is not well understood. In this thesis, a perturbation approach is introduced to derive a “ ” (GS) of subtidal along-channel velocity profile for estuarine circulation incorporating the effects of tidal and lateral processes, which allows us to evaluate the possible circulation patterns, the underlying mechanisms, and to diagnose the contributions of these nonlinear processes. The GS applies to the entire estuary at any location without including presumptions of constant baroclinic pressure gradient and constant eddy viscosity. The GS was tested against a numerical model simulation case. A non-dimensional parameter  is introduced for determining the structure of estuarine circulation. The success of HR65 has been evaluated and compared with GS. It is found that HR65 is a particular case of GS when estuarine circulation is dominated by horizontal pressure gradients and it has a good predictive skill when X is in the range between -0.53 and -0.20. However, its scaling of the strength of estuarine circulation could deviate from the true circulation when these nonlinear effects are important unless this scaling or its dependence on constant eddy viscosity is rectified by a scaling factor Y . GS can be considered as the superimposition of flow uc that is only related to depth-averaged velocity C and exchange flow ur. The exchange flow consists of six possible patterns. Four of them have two-layer structure, and the remaining two have xii three-layer structure. The pattern of the estuarine circulations will be altered and some patterns cannot be observed when the magnitude of C is comparable to exchange flow. The circulation exhibits a total inflow (or outflow) when C is much large than exchange flow. The typical pattern of circulation is the two-layer circulation with seaward flow in the upper layer and landward flow in the lower layer. Both the lateral processes and tides could significantly change the pattern through its effect on mixing and barotropic forcing. Although terms higher than 4th-order can reduce the deviation of model results from the real circulation, their inclusion does not have clear physical meaning. Therefore, the general solution only retains a 4th-order approximation of the circulation that can explain both two-layer and three-layer circulation patterns being observed in estuaries. Besides the density-driven circulation, other possible circulations such as tideinduced and laterally-induced circulation are discussed. It is found that they could create two-layer circulation that has structure similar to observations. However, the results suggest that baroclinic forcing is the primary fundamental mechanism of estuarine circulation.

The hydrology of the 335 km 2 catchment of the Knysna River is described using several standard techniques, including those described in Hydrological Research Unit Reports Nos 2/76, 1/72 and 2/78. The mean annual . 6 -1 run-off at the farm Charles ford is about 61 x 10 m3 an. . Salinity measurements were made at various stations between January and June 1984. These data were used to calibrate and verify a 1-dimensiona~ mathematical model of the Knysna Estuary. The model showed that the salinity would rise to 36 parts per thousand at the Old Drift if the · river flow was stopped altogether. The model also shows that salinities drop to 1 or 2 parts per thousand at the National Road Bridge for three successive tides following a one-in-two-year flood -1 ( 40 m 3 s ) • The response of the biota to changing salinities is described using available literature. It appears that there will be a gradual decline in primary and secondary productivity if the river flow is reduced. Other management approaches to this problem as well as the ethical problem of reducing the freshwater inflow into an estuary are explored.

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