Introduction

The Mediterranean climate of coastal southern California has induced some obvious and distinct contrasts between mesic and xeric vegetation-types. Coastal sage scrub, chaparral, oak woodland, or California grassland vegetation often ends abruptly at a narrow corridor of riparian vegetation. Woody, perennial wetland vegetation is usually confined to a relatively narrow corridor bordering the enduring, year-round, low-volume water flows and is not directly correlated with the mean annual or ephemeral high-volume storm flows. Consequently, unmodified floodplains in southern California are often relatively wide when compared with the narrow strip of riparian vegetation which frequently occurs only along the path of low flow.

The Mediterranean climate may, in part, have promoted human occupancy of southern California floodplains by fostering the false impression, for decades at a time, that riparian growth delimited the floodplain. Following the rare but inevitable devastating flood, the typical human response has been to "improve" the floodplain to accommodate this rare flood. In doing so, headwaters are dammed, and the natural floodplain usually is constricted into a channelized floodway in order to provide protection for floodplain developments. To maximize hydraulic efficiency, riparian vegetation is typically removed. The trapezoidal, concrete channel of the Los Angeles River is a famous example of such maximized hydraulic efficiency.

Since more than half of all Californians live in the four coastal counties of southern California (Ventura, Los Angeles, Orange, and San Diego), unmodified riparian corridors have largely been obliterated. Some significant areas of riparian vegetation do exist, particularly in San Diego County. In some San Diego County rivers, such as the Mission Valley region of the San Diego River, historic sand mining has brought riverbottom elevations nearer to the water table, which facilitated marsh and riparian woodland establishment once mining ceased. Also, the reestablishment of perennial freshwater flows by irrigation and wastewater returns encouraged wetlands redevelopment. However, riparian wetlands continue to be threatened by human actions. Using actual case histories, we will attempt to document the major threats to coastal San Diego County's riparian resources and the effectiveness of agencies in protecting these resources; and we will offer some general observations as to what factors influence the effectiveness of attempts to protect riparian systems.