Date of Award
Doctor of Philosophy in Environmental Sciences
Natural Resources Science
Natural Resources Science
Arthur J. Gold
Rivers and river systems serve as conduits for nutrients and organisms, function as corridors for fish and wildlife passage, and provide resources for humans. Streamflow has been called the master variable in a river because it affects habitat diversity and availability through its impact on physical factors that influence habitat quality. However, land use changes such as urbanization and irrigation, can have major effects on stream hydrology. Modifications of the land surface due to urbanization alters natural stream hydrographs by increasing flood peaks, decreasing time to peak flows, and causing higher runoff velocities. Irrigation may produce the opposite effects.
In order to preserve a spectrum of stream functionality, rivers must maintain seasonally adequate flows. For example, low flows can affect stream connectivity, restrict movement of aquatic organisms, concentrate prey into limited areas, purge invasive species from riparian corridors, and enable recruitment and evolution of floodplain plants. State agencies throughout the Northeast U.S. are considering policies linked to low-flow thresholds that sustain these ecosystem services. Methods that set minimum flow standards often result in conflicting values, due to differing environmental goals and levels of protection they aim to achieve. Two such methods, the USFWS Aquatic Base Flow (ABF) method and the Wetted Perimeter method have been widely used. The USFWS ABF method recommends using the median of August flows and has been refined for Rhode Island (RIABF). The wetted perimeter method uses stream cross-sections at riffle locations to determine critical flow values to maintain flow based on the wetted perimeter of the channel. In addition to setting flow standards, methods to minimize the adverse effects of urbanization have also been proposed. Low impact development (LID) has emerged as a strategy to reduce the hydrologic impacts of urbanization on aquatic ecosystems by combining site planning and design processes with runoff reduction and treatment practices.
Within a given climatic region, water resource managers seeking to optimize stream ecosystem services need a clear understanding of the importance of land use, physical/climatic characteristics, and hydrography on different components of stream hydrographs. Within 33 Southern New England watersheds (average area 80 km2), we assessed relationships between watershed variables and a set of low flow parameters: 1-, 7- and 30- day minimum flows. We used an information theoretical approach to develop regression models to identify relationships between landscape attributes and parameters that describe different components of the flow regime. The key variables identified by the AIC weighting factors as generating positive relationships with median annual minimum flow events included percent stratified drift (greater infiltration and storage), mean elevation (likely related to higher snowfall), drainage area and mean August precipitation. The extent of wetlands in the watershed was negatively related to low flow magnitudes likely due to the capacity of those ecosystems to remove water from the basins via evapotranspiration during drought conditions. Of the various land use variables, the percent developed land was found to have the highest importance, but it was less important than wetlands and physical/climatic features. The extent of impervious cover in the study watersheds was primarily less than 10% and the study watersheds were generally larger than watersheds used in other studies relating impervious cover to stream health. Our results suggest that even with watersheds located within close spatial proximity, strategies focused on balancing water extraction to sustain low flows in fluvial systems can benefit from attention to select watershed features. We draw attention to the finding that streams located in watersheds with high proportions of wetlands may require more stringent approaches to withdrawals to sustain these ecosystems during drought periods.
We then determined the minimum flow requirements at three locations (riffle zones) along the Beaver River, located in southern Rhode Island, using both the wetted perimeter method and the RIABF method. In order to determine stream flow at ungaged locations, runoff was modeled using the HEC-HMS rainfall/runoff model.
To assess biological conditions, we reviewed macroinvertebrate, fish and temperature data obtained within the watershed. Biological conditions of the Beaver River indicated that the Beaver River is a well-functioning stream habitat.
Minimum stream flow requirements using ABF and WP methods were investigated. Stream flows were found to be below the ABF value between six to 21% of the time and below the WP flow between 37 to 72% of the time.
Physical and biological sampling done in the watershed indicate the river is a well-functioning, river, comparable to pristine sites; however minimum flow criteria set by the wetted perimeter method suggest that the river is flowing below critical flow values over 50% of the time. Our results suggest that minimum flow values obtained from the wetted perimeter method for southern New England rivers should be approached with caution and should be compared to results obtained from other methods to determine the accuracy and applicability of the critical flows prior to using these values for any type of instream flow regulations.
We also assessed the effect of increased impervious cover for both conventional and LID-based urbanization on low flow metrics and flow depths in riffle habitats in a small, relatively undeveloped watershed located in southern Rhode Island. We employed a hydrologic model to simulate stream flow, base flow and storm flows under different land cover scenarios and then compared these results to the effects of direct stream withdrawals from agricultural irrigation.
We found baseflow to be negatively correlated to impervious area. On pervious surfaces, direct runoff is likely to be infrequent during the summer months, when most of the precipitation that falls is utilized for the soil moisture deficit. In contrast, connected impervious area (IA) will generate immediate runoff to streams from rainstorms that would have otherwise infiltrated the soil. During periods of excess precipitation, the falling limbs of those hydrographs generated prolonged periods of comparatively elevated flows.
Combining baseflow and storm flow shows that increased values of IA can generate higher flow values during the summer months during periods with excess precipitation. As IA increases through the different land use scenarios, storm related runoff increases immediately following precipitation events, causing higher stream flows. The small decreases in base flow input to the stream due to increased IA are negated by the impacts of the higher storm flows, causing summer stream flows to be higher under the developed land use scenarios than existing conditions. Changes to the channel depth of the riffles were also relatively minor.
During a year with median precipitation, the model predicted a lower frequency of low flows with both conventional development and with LID compared to the predictions for the limited development present in current conditions. Both conventional development and LID also display fewer low flow periods during a dry year, but the pattern reverses, with LID predicted to have slightly lower frequencies of low flows than the conventional development. Over the summer, storm runoff and the associated falling limb of the runoff hydrograph that results from connected impervious cover occurs with enough frequency to influence the low flow thresholds we use for metrics. During the dry year, rainfall occurrences were very infrequent and the higher baseflow associated with LID accounts for the slight increase in flows compared to the conventional development. Irrigation scenarios decreased both flows and depths. Changes in land use generally increase river flows while water withdrawals decrease river flows. The occurrence of low flows within the Beaver River was found to be relatively resilient to the extent of development and water withdrawals simulated by this study.
These analyses will help inform future water management decisions in watersheds with the diversity of land uses that occur in southern New England.
Morrison, Alisa, "Sustaining Environmental Flows in Southern New England Rivers: Effects of Watreshed Factors and Land Use" (2013). Open Access Dissertations. Paper 61.