Research Areas > Wetlands > Coastal Watershed Historical Ecology: San Gabriel River

Project: Coastal Watershed Historical Ecology: San Gabriel River

Background and Objectives

The Southern California Coastal Water Research Project (SCCWRP), in cooperation with its technical partners (below) and the Southern California Wetlands Recovery Project (WRP), has developed a regional historical ecology program focusing on coastal wetlands and watersheds in the Southern California Bight. This program represented SCCWRP’s first historical ecologic analysis and is one element of the WRP’s larger regional wetland monitoring and assessment program. The project established a framework for understanding the historical extent and function of Southern California wetland and riparian resources, including how they have changed through recent human history via an analysis of the floodplain of the San Gabriel River. The San Gabriel River provided an excellent pilot location for this work because there have been dramatic physical and cultural changes to the watershed, the social history is fairly well documented. In addition, the watershed is currently the subject of several integrated resource management and planning efforts. The goal of this project was to use a variety of historical resources to answer the following questions:

1) What was the historical acreage and distribution of wetland and riparian habitat in the watershed?

2) What was the historical extent and composition of wetlands in the floodplain?

3) What was the general composition and spatial distribution of riparian vegetation community types and species?

4) To what extent has the wetland and riparian habitat changed over time and what factors have been associated with these changes?

Status

This project was completed in 2007.

Methods

Numerous data sources were used to gain insight into the historical wetland and riparian habitats. Primary data sources included Mexican land grant sketches (diseños) and US General Land Office maps from the 1850s, irrigation maps from the 1880s, topographic maps and soil surveys from the early 1900s, and aerial photographs from the 1920s. Secondary data sources included oral histories, essays, ground photographs, and field notes. Data sources were digitized, georeferenced, and overlaid in GIS to produce historical wetland polygons. Polygons were then (1) attributed for data sources, (2) classified using the US Fish and Wildlife Service National Wetland Inventory (NWI) system to facilitate comparison with contemporary conditions, and (3) assigned a confidence rating based on the certainty in the primary data sources. The concordance between multiple data sources allowed us to draw conclusions based on the collective “weight of evidence” that supported inferences about historical condition. The resulting maps were analyzed for historical extent and distribution of wetlands and riparian habitat and compared to contemporary wetland maps to assess wetland losses. Finally, historical herbaria records and bird observations were used to confirm the results of the GIS analysis and to provide insight into the composition of historical wetland plant communities.

Findings

The period of investigation for this study was characterized by higher than average rainfall and streamflow resulting from multi-decadal climatic cycles that produced wetter than average weather patterns between 1750 and 1905. The frequency and magnitude of extreme runoff events, combined with the lack of human intervention (in the form of ground water extraction, flood control, dams, or diversions) resulted in a highly dynamic river system that supported extensive wetland complexes interspersed among the upland floodplain habitats. Throughout the early 19th century (and likely during earlier periods), the path of the San Gabriel River oscillated between functioning as a tributary to the Los Angeles River and assuming one of several distinct flow paths to the ocean. Following a series of large floods during the 1860s, the river assumed a course similar to its contemporary alignment. Therefore, the circa 1870 period was chosen as the focus of the investigation.

Based on estimates, the San Gabriel river floodplain supported at least 47,000 acres (19,000 ha) of wetlands, with palustrine alkali meadows being the most common wetland type. Following wet years when the river overflowed its banks, the floodplain likely supported an additional 800-4,000 acres (300-1,600 ha) of wetlands as part of seasonal wetland/upland complexes.

Development of the San Gabriel River watershed over time has resulted in extensive wetland losses. This study estimated that greater than 86% of historical wetlands have been lost since ca. 1870. Palustrine wetlands have been particularly impacted, with most of the perennial and intermittent ponds and marshes no longer present. Of particular note is the loss of the vast alkali meadows, which were once the most common type of wetland in the lower watershed, but are now totally absent from the landscape. Channelization and other flood control measures have resulted in conversion of the meandering and braided channel systems to linear flood control conduits. Similarly, the complex of seasonal floodplain wetlands has been almost entirely lost.

Despite these dramatic wetland losses, several opportunities exist for wetland restoration. Remnant wetlands and/or wetland signatures exist at locations such as Whittier Narrows, along the base of the foothills of the San Gabriel mountains, in the upper floodplain, and at a several locations in the Long Beach area. Knowledge of landscape positions and wetland types that previously existed can help guide decisions regarding future restoration of these areas. Furthermore, the reconstructed plant community compositions generated by this study can provide templates for restoration planning. However, caution must be taken in the use of historical information. Restoration of wetland plant communities to their former historical configuration may not be possible for several reasons, including irreversible alteration of hydrology or soils. Thus historical analysis must be used to inform, but not replace the tools commonly used in watershed restoration science.

Partners

California State University Northridge (CSUN)
San Francisco Estuary Institute (SFEI)
University of California, Los Angeles (UCLA)
University of Southern California (USC)
Santa Monica Bay Restoration Commission
The Los Angeles and San Gabriel Rivers Watershed Council