The effects of watershed urbanization on the stream hydrology and riparian vegetation of Los Peñasquitos Creek, California
Introduction
There has been considerable research concerning the consequences of altering riverine hydrology (see review in Poff et al., 1997), and there is increased interest in urbanization as a source of hydrological alterations (Paul and Meyer, 2001). Given the close coupling of stream hydrologic characteristics and riparian plant species ecology (Stromberg, 1993, Stromberg, 1998, Scott et al., 1996, Poff et al., 1997, Mahoney and Rood, 1998, Shafroth et al., 1998), we examined the effects of watershed urbanization on riparian vegetation communities via alterations in the hydrologic regime of a coastal Southern California stream system.
Urbanization within a watershed increases the area of impervious surfaces (Paul and Meyer, 2001), which decreases infiltration of precipitation and increases runoff (Dunne and Leopold, 1978, Gordon et al., 1992, Leopold, 1994). Runoff increases in proportion to the cover of impervious surface in a watershed (Arnold and Gibbons, 1996), and the increased storm runoff increases peak discharges and flood magnitudes (Dunne and Leopold, 1978). Increases in flood magnitudes are greater for floods of shorter recurrence intervals than those with long recurrence intervals (Hirsch et al., 1990). Reduced infiltration of precipitation to groundwater aquifers may reduce groundwater recharge and stream base flow (Paul and Meyer, 2001). However, importing water into an urban watershed for landscape irrigation may increase stream base flow (Hirsch et al., 1990, Paul and Meyer, 2001, Greer and Stow, 2003).
Recent research has demonstrated the intimate relationship of riverine hydrology and fluvial processes and riparian plant species recruitment and survival (Scott et al., 1996, Scott et al., 1997, Shafroth et al., 1998, Stromberg, 1993, Stromberg, 1998). Riparian plant species establish in locations where there are suitable conditions for seed germination and sufficient water for seedling survival, and where the species can tolerate physical disturbance from floods (Stromberg and Patten, 1992, Hupp and Osterkamp, 1996, Scott et al., 1996, Mahoney and Rood, 1998). Thus, the structure of riparian vegetation communities is often a mosaic of species and age class composition produced by spatial and temporal variations in stream discharge patterns (Stromberg et al., 1997, Auble and Scott, 1998, Shafroth et al., 1998).
Poff et al. (1997) discuss the concept of “natural flow regime” of riverine systems, which they characterized as the magnitude, frequency, duration, timing, and rate of change of discharge. The natural flow regime determines the species composition and spatial patterns of riverine biological communities; thus, modifications to the natural flow regime, as a result of river regulation and impoundments, have well-documented effects on riparian vegetation communities (Harms et al., 1980, Conner et al., 1981, Hunter et al., 1987, Stromberg and Patten, 1992, Stromberg, 1993, Stevens et al., 1995, Poff et al., 1997). However, there has been little research published on the influence of urbanization-induced hydrologic changes on riparian vegetation communities (Poff et al., 1997, Paul and Meyer, 2001). Because urbanization of watersheds can modify the natural flow regime of stream systems, it is expected that riparian vegetation communities ultimately would be affected by urbanization as well.
We evaluated the potential effects of watershed urbanization on a coastal stream and associated riparian vegetation community. Our research objectives were to (1) quantify the urbanization of a coastal Southern California watershed; (2) evaluate potential urbanization-induced changes in streamflow characteristics in this watershed; (3) quantify temporal changes in the distribution of the riparian vegetation community in this watershed; (4) evaluate whether these distribution patterns are consistent with observed hydrologic changes.
Streams and associated riparian vegetation are important natural resources in the urbanizing areas of Southern California and are a focus of conservation and land management activities. Many local governments in Southern California are conducting land use planning efforts that attempt to balance urban development needs with natural resources protection. These planning efforts often target streams and riparian communities for preservation, but generally do not consider the indirect effects of watershed urbanization on hydrologic processes. As these indirect effects can change the character and quality of stream and riparian habitats, land use plans must consider how watershed-scale processes, such as hydrologic regimes, control the characteristics of riverine resources and how they are affected by land use changes. For conservation of riverine resources to be successful, we must develop a better understanding of the dynamics of these habitats, in the face of urbanization, to inform land planning and management.
Section snippets
Area description
Our study was conducted in the Los Peñasquitos Creek watershed, San Diego County, California (Fig. 1A). The Los Peñasquitos Creek watershed encompasses approximately 15,759 ha, spanning the area from Iron Mountain (elevation 822 m above mean sea level) on the east to Los Peñasquitos Lagoon at the coast. Coastal sage scrub and chaparral characterize the vegetation communities on the upland slopes of the watershed, and oaks intermingle with sycamores and willows along the margins of streams and in
Cattle grazing
Cattle were first introduced into the lower basin of the Los Peñasquitos Creek watershed in 1823, and, presumably, settlers altered the riparian vegetation community by clearing trees for firewood and charcoal and to create agricultural fields in floodplain areas (Pourade, 1969). Cattle were grazed continuously in the lower Los Peñasquitos Creek watershed until 1989, when they were removed to eliminate conflicts between ranging cattle and increasing vehicular traffic in the area. During the
Discussion
Previous research has estimated runoff increases of 200–500% where impervious surface cover exceeds 10% of the watershed (Arnold and Gibbons, 1996, Paul and Meyer, 2001). The results of this study are consistent with these findings. As urbanization in the Los Peñasquitos Creek watershed increased from 9% to 37% (Fig. 2), total runoff increased by an average of 4% per year (Fig. 4), representing an increase of over 200% from 1973 to 2000. During the same time period, there was no change in
Conclusions
We assert that the increase in urbanization of Los Peñasquitos Creek watershed has resulted in (1) significant increases in annual median and minimum discharges and dry season runoff; (2) increases in flood magnitudes; (3) geomorphic changes to stream channel morphology; (4) hydrologic changes favoring the expansion of a willow-dominated riparian vegetation community. We attribute the changes to an increased conveyance of storm runoff from the greater impervious surface area and increased
Acknowledgements
We gratefully acknowledge the San Diego Foundation Blasker-Rose Miah Fund for their generous support of this project (Grant #C-2001-00551). We would like to thank Stuart Hurlbert, Thomas Oberbauer, and five anonymous reviewers who provided valuable comments. We would especially like to thank Jerre Ann Stallcup for her contributions and comments.
Michael D. White received his Ph.D. from the San Diego State University and University of California, Davis Ecology Joint Doctoral Program. He is a senior ecologist at the Conservation Biology Institute, where he is working on large-scale natural resources conservation and management plans. Dr. White is an adjunct associate professor in the Biology Department and a faculty associate at the Center for Inland Waters at San Diego State University. His research interests involve exploring the
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Michael D. White received his Ph.D. from the San Diego State University and University of California, Davis Ecology Joint Doctoral Program. He is a senior ecologist at the Conservation Biology Institute, where he is working on large-scale natural resources conservation and management plans. Dr. White is an adjunct associate professor in the Biology Department and a faculty associate at the Center for Inland Waters at San Diego State University. His research interests involve exploring the interrelationships of hydrological and biological characteristics and how they are influenced by land use and water management practices.
Keith A. Greer is the deputy planning director for the City of San Diego. He has been involved in environmental planning for the past 14 years. Keith manages the review of large land development projects, including policy development, conflict resolution, and regulatory permitting. He was a key member of the team that helped design the City of San Diego's Multiple Species Conservation Program and is now overseeing its implementation, including land acquisition and biological monitoring. Keith holds a bachelors of science in biology and a masters in geography, with a concentration in Natural Resources and Environmental Policy.