Research Areas > Wetlands > Coastal Watershed Historical Ecology: Ballona Creek and Lower Ventura and Santa Clara River

Project: Coastal Watershed Historical Ecology: Ballona Creek and Lower Ventura and Santa Clara River

Background and Objectives

Vast resources are devoted to watershed management, resource protection, and wetland restoration. Historical wetland losses are often cited as a motivation for prioritizing ambitious wetland restoration and management activities. However, analysis of historical conditions is often underutilized in the planning process. Historical ecology can be a valuable tool to help understand the mechanisms of past decline, provide templates for future restoration, and provide context for making decisions about allocation of scarce resources. For example, historical analysis provides insight into where our greatest losses have occurred in terms of geography and specific habitat types. It also helps us to recognize the controlling factors and constraints affecting local habitats, and how they have changed or stayed the same over time.

The overall goal of this project was to provide new understanding about a given historical baseline condition of streams and wetlands across southern California’s coastal watersheds based on information from the mid- to late-19th century through the early 20th century. This project compiled sentinel data sets on historic condition, and used these data to evaluate how the distribution of wetlands has changed over time in response to key changes in land use or stream management. The changes examined included distribution of wetland and riparian habitat in the watershed during the period from 1850-1910, structure and composition of riparian habitat over time, riparian structure of the floodplain in wet vs. dry years, and spatial distribution of wetland and riparian vegetation community types and wildlife species.

Status

This project was initiated in 2007 and completed in 2011.

Methods

Unlike contemporary habitat analysis, historical analysis relies on interpretation of multiple data sets that were not collected to meet the objectives of our analysis. Therefore, conclusions must be developed from multiple data sources that collectively provide a “weight of evidence” that supports inferences about historical condition. The process consisted of the following general steps:

1) Compile primary data sources on historical condition, including: maps, soil and geologic surveys, and aerial photography. Sources were considered ‘primary’ if they were collected using a structured procedure that allowed for general quality control at the time they were produced.

2) Use primary data sources to refine study area and to prioritize locations and time periods for additional analysis.

3) Compile secondary data sources that expand on or clarify primary sources. Secondary sources include personal and written accounts, ground-level photographs, and floral and faunal records.

4) Digitize and georeference acceptable data sources.

5) Overlay all suitable data sources to create an initial set of historical wetland polygons.

6) Classify inferred historical wetland polygons using a system that is compatible with the contemporary National Wetlands Inventory (NWI) mapping system. This allows comparison of historical and current wetland extent and distribution.

7) Assign confidence levels to the wetland polygons based on estimated accuracy and concordance of available data sources.
 

Findings

The second study of the Ventura River valley, lower Santa Clara River valley, Oxnard Plain, and Ventura County shoreline revealed an ecologically diverse landscape. Valley floor habitats were relatively dry overall, with extensive open grasslands and scrublands. Live oaks and sycamores colonized terraces in the Ventura River valley, in addition to many alluvial fan surfaces north of the Santa Clara River. With few exceptions, non-riparian wetlands were concentrated on the Oxnard Plain. Wetland distribution was largely shaped by the migration of the Santa Clara River over geologic time. This migration also created a pattern of coastal lagoon systems along the shoreline, leaving a legacy of perched and closed lagoons marking former river mouths. In addition, at least three types of coastal estuarine systems are represented on the Ventura shoreline. These features formed a near continuous sequence of coastal wetlands from Mugu Lagoon all the way to the Ventura River mouth. These findings provide insight into the dynamics and processes that shaped the Ventura landscape to help inform the goals and values of restoration.

Findings from Ballona Creek watershed showed the area supported a great diversity of wetlands during the mid-late 19th century. The La Cienega wetlands and the Ballona Lagoon complex accounted for the majority of wetland area in the watershed. Throughout the watershed, the dominant wetland types included alkali meadow (35%), valley freshwater wet meadow (10%), valley freshwater marsh (10%), brackish to salt marsh/tidal marsh (9%), and alkali flats (8%). In addition, 232 miles (373 km) of historical stream channels were mapped in the study area. Approximately 80% of the stream channels were intermittent (often discontinuous) washes. Across the valley floor most of the streams sank into porous soils or spread into the major wetland complexes of La Cienega and the Ballona Lagoon. This likely contributed to a significant amount of subsurface water flow and to the vast wetland complex at La Cienega. Freshwater seeps and springs were also a characteristic feature of the Ballona Watershed. Although springs were present at a few locations throughout the Ballona Valley, 70% of the 45 mapped springs in the watershed were found in the Santa Monica Mountain foothills. Many of these springs persist today and are unique remnant features from the historical landscape.

Partners

California State University Northridge (CSUN)
San Francisco Estuary Institute (SFEI)
University of California, Los Angeles (UCLA)
University of Southern California (USC)