Hydrologic Changes in Urban Streams and Their Ecological Significance

—Urban development modifies the production and delivery of runoff to streams and the resulting rate, volume, and timing of streamflow. Given that streamflow demonstrably influences the structure and composition of lotic communities, we have identified four hydrologic changes resulting from urban development that are potentially significant to stream ecosystems: increased frequency of high flows, redistribution of water from base flow to storm flow, increased daily variation in streamflow, and reduction in low flow. Previous investigations of streamflow patterns and biological assemblages provide a scale of ecological significance for each type of streamflow pattern. The scales establish the magnitude of changes in streamflow patterns that could be expected to produce biological responses in streams. Long-term streamflow records from eight streams in urbanizing areas of the United States and five additional reference streams, where land use has been relatively stable, were analyzed to assess if streamflow patterns were modified by urban development to an extent that a biological response could be expected and whether climate patterns could account for equivalent hydrologic variation in the reference streams. Changes in each type of streamflow pattern were evident in some but not all of the urban streams and were nearly absent in the reference streams. Given these results, hydrologic changes are likely significant to urban stream ecosystems, but the significance depends on the stream’s physiographic context and spatial and temporal patterns of urban development. In urban streams with substantially altered hydrology, short-term goals for urban stream rehabilitation may be limited because of the difficulty and expense of restoring hydrologic processes in an urban landscape. The ecological benefits of improving physical habitat and water quality may be tempered by persistent effects of altered streamflow. In the end, the hydrologic effects of urban development must be addressed for restoration of urban streams. * Corresponding author: [email protected] Introduction Streamflow is the dominant physical process distinguishing rivers and streams from other ecosystems. The structure and composition of lotic communities in fluvial ecosystems depend on source, timing, and rate of streamflow as they regulate both habitat conditions and disturbance regimes. Because of the important roles of streamflow in fluvial ecosystems, hydrologic changes in urban streams pose plausible and potentially significant ecological risks that warrant evaluation, along with other recognized changes (e.g., water chemistry, physical habitat, riparian conditions). Although previous investigations have associated degradation in the biological condition of streams with urban development (Klein 1979; Wang et al. 2000; Morley and Karr 2002; Booth et al. 2004), streamflow patterns per se are seldom implicated as a primary cause of urban stream degradation. Failure to address the significant hydrologic changes in urban areas may limit the success of restoration efforts in rivers and streams. “Urbanization” is not a single condition; instead, it is a collection of actions that lead to recognizable landscape forms and, in turn, to changes in stream conditions. No single change defines urbanization, 158 KONRAD AND BOOTH but the cumulative effect of human activities in urban basins profoundly influences streams and their biota. Karr and Yoder (2004) provide a conceptual framework for assessing biological degradation in which the human actions that constitute urbanization modify many aspects of stream ecosystems, which elicit biological responses (Figure 1). Among the many aspects of stream ecosystems modified by urbanization, changes in hydrologic regimes (i.e., the timing and rate of streamflow) and potential biological responses are the focus of this paper. The influence of hydrologic modification, however, is likely to depend on a broader ecological context, and so the importance of hydrologic modification is likely to vary from stream to stream and region to region. We focus on hydrology as a primary link between urban development and biological responses in urban streams because of the well-established influence of streamflow on the structure, composition, and productivity of lotic communities. In this paper, we briefly review the literature on relations between streamflow patterns and lotic communities to identify the types of streamflow patterns that have demonstrable effects on stream biota. We then extend the results of two investigations, Poff and Allan (1995) and Clausen and Biggs (1997), that associated variation of biological assemblages with variation in a variety of streamflow statistics. Our extended analyses identify the magnitude of variation in four streamflow patterns (high flow frequency, flow distribution, daily variation, and low flow magnitude) that was associated with biological differences within their respective groups of streams. The magnitude of hydrologic variation associated with biological differences establishes the scale of ecological significance for assessing changes in each streamflow pattern. We then analyze long-term streamflow records from eight urban streams and five reference streams in the United States, where land use has been relatively stable, to identify streamflow patterns that changed in association with urban development. Finally, we discuss our results in the context of managing urban streams. Hydrologic Effects of Urban Development Hydrologic effects of urbanization can be characterized as the redistribution of water once intercepted by Aspects of stream ecosystems