Spatial and temporal characteristics of groundwater levels adjacent to beaver ponds in Oregon Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/ws859h582

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  • This study was undertaken to evaluate the spatial and temporal characteristics of riparian groundwater tables adjacent to beaver ponds. The research was conducted in two parts; in the first portion a two-dimensional, finite-difference computer model was developed and utilized to simulate groundwater elevations through time in pond-adjacent aquifers. Two types of synthesized riparian settings, inhabited by beaver, were used as model sites: a stable, effluent stream reach located in the humid Coast Range in western Oregon and an incised, influent stream reach located in the semi-arid region east of the Cascade Range in central Oregon. Within the context of regional site parameters a range of input values were used to determine the effects of alternative hydrologic and hydraulic site conditions on groundwater tables. These input data included water level gradients, substrate hydraulic conductivity and storativity values and sources and magnitudes of recharge and discharge. The second part of the investigation was a case study in which groundwater levels next to a beaver pond located within a western Oregon stream reach were monitored from July 1991 through July 1992. The results from both parts of the study indicated that beaver ponds cause elevated groundwater tables and increase the dimensions of the subsurface saturated soil zone in adjacent riparian areas. The simulation results included small, intermediate and large dimensions of the pond-adjacent saturated wedge per month throughout a one year period for each of the two synthesized riparian aquifer systems. The maximum vertical and lateral extents and the saturated soil and stored groundwater volumes were calculated for each pond-adjacent wedge. The quantitative bounds of the saturated wedge, including all sites and cases, ranged between 0.3 m high and 4 m wide to 1.7 m high and 90 m wide. Similarly, the volume of the pond adjacent saturated soil wedge ranged between 12 and 3500 m³; the associated volumes of stored groundwater ranged from 0.6 to 700 m³. Beaver ponds were found to contribute to greater spatial and temporal hydrologic diversity for riparian groundwater conditions compared to those without pond influences. The magnitude and/or direction of groundwater flow between each pond and adjacent riparian aquifer was influenced by the presence of a pond. These changes occurred in response to a hydraulic differential created by the beaver pond within each groundwater system; the pond caused spreading of subsurface flow, acted as a hydraulic control or induced a combination of both of these conditions and created a localized flow cycling zone between the stream and riparian aquifer. Also, seasonal fluctuations in each pond-adjacent saturated wedge occurred due to climate dependent changes in inflow and outflow rates to and from the aquifer. Sensitivity analysis indicated that the substrate hydraulic conductivity and the head difference between the pond surface and the stream surface below the dam had the greatest effect on the pond-adjacent groundwater levels. The expansion of the saturated zone was directly proportional to the dam height and to the aquifer transmissivity. The pond and riparian aquifer interactions demonstrate a significant dimension in the ecology of beaver inhabited streams and should be included in holistic constructs of riverine ecosystems. Elevated pond-adjacent groundwater tables and greater exchange between surface and ground waters can promote vegetation growth and related sediment deposition and channel stability, increase nutrient transformations and biogeochemical pathways, influence soil genesis and morphology, increase biodiversity and create connectivity within the stream-riparian continuum.
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