The effects of watershed urbanization on stream hydrologic characteristics and riparian vegetation of Los Peñasquitos Creek

July 2002

There is increasing interest in the effects of urbanization on natural systems. Among the variety of observed effects, urbanization has long been recognized as affecting hydrological characteristics of streams and rivers. Given the close coupling of stream hydrologic characteristics and riparian plant species ecology, we examined the effects of watershed urbanization on riparian vegetation communities via alterations in the hydrologic regime of a coastal southern California riparian system. These coastal river systems in southern California have received little attention in the literature.

Urbanization within a watershed increases the area of impervious surfaces. Increasing the area of impervious surfaces generally decreases infiltration of rainfall and increases runoff. Runoff increases in proportion to the cover of impervious surface in a watershed, and increased runoff from storms increases peak discharges and flood flows. However, floods with long recurrence intervals may be less affected than floods of shorter recurrence intervals. Reduced infiltration of precipitation to groundwater aquifers may reduce groundwater recharge and stream baseflow. It is possible that importing water into an urban watershed for landscaping irrigation may offset the reduced groundwater recharge, thereby mitigating potential reductions of stream baseflow.

Recent research has demonstrated the intimate relationship of riverine hydrology and fluvial processes and riparian plant species recruitment and survival. The distribution of many native riparian plant species along streams in the southwestern United States is a function of the autecology of these species relative to stream hydrology. Woody riparian plant species establish in positions along streams where there are suitable conditions for seed germination and sufficient water for seedling survival, and where the species can tolerate physical disturbance from floods. Thus, the structure of riparian vegetation communities is often a mosaic, at varying spatial scales, of species and age class composition produced by spatial and temporal variations in stream discharge patterns.

Poff et al. (1997) have discussed the concept of the "natural flow regime" of riverine systems as the critical determinant of their biological structure. The flow regime can be described by five important characteristics: magnitude, frequency, duration, timing, and rate of change of discharge. Modifications to the natural flow regime by river regulation and impoundments have well-documented effects on riparian plant species and vegetation communities.

However, there has been little research published on the influence of urbanization-induced hydrologic changes on riparian vegetation communities. As urbanization of watersheds can modify the natural flow regime of stream systems, it is expected that riparian vegetation communities would be affected as well.

Our study was conducted in the Los Peñasquitos Creek watershed, San Diego County, California. The objectives of this research were to:

1. Document the urbanization of the watershed over time;

2. Describe the hydrologic characteristics of Los Peñasquitos Creek, and evaluate potential urbanization-induced changes in these characteristics; and

3. Map and quantify changes in the distribution of the riparian vegetation community in the lower Los Peñasquitos Creek watershed over time, and evaluate whether these distribution patterns are consistent with observed hydrologic changes.

For more information, contact Jerre Stallcup (email: jastallcup@consbio.org) or Mike White (email: mdwhite@consbio.org) in the CBI San Diego office (phone: 760-634-1590).