Waterbird response to modified hydrology
of Central Valley wetlands

Wetlands of California’s Central Valley have historically been home to one of the largest concentrations of waterbirds in North America. However, California has lost more than 90% of its original wetland area.  Therefore, remaining wetlands are extremely important to waterbird populations. Wetland management determines habitat structure, composition and productivity, which affects waterbird use. 

The Grassland Ecological Area (GEA) in Merced County, CA embodies the largest contiguous wetland remaining in California. These wetlands are highly modified and intensively managed in order to produce sufficient food and cover for wintering wildlife, especially waterfowl. Food generated for wildlife in the area consists mainly of moist-soil managed plants, primarily swamp timothy (Crypsis schoenoides). 


Wetlands managed for swamp timothy (left) and watergrass (right):
efore flooding (top, fall); Flooded (center, winter); and after drawdown (bottom, spring).

The wetlands of GEA drain into the San Joaquin River (SJR), where salinity levels have become a concern. Therefore, wetland managers were requested to delay drawdown of their wetlands by 4-6 weeks, so that highly saline water enters the SJR later in the year when there is enough flow from reservoir releases upstream to dilute the salts discharged. However, wetland managers are concerned that a change in the hydrological regime may cause retention of salts, negatively impacting wetland plant productivity, and ultimately reduce food availability for waterbirds that rely on the GEA for non-breeding habitat.

grasslands ecological area

This study, led by graduate student Melissa Odell, aimed to assess the response in avian use, from late-winter through spring, of wetlands subjected to delayed drawdown during 3 years. She quantified differences in wetland use by geographic location (block), management type, season, and year for waterbird species richness and four functional waterbird groups (dabbling ducks, surface-diving birds, shorebirds, and large wading birds).


Seasonal trends in dabbling duck and shorebird density during each year of waterbird surveys.

Each point represents the mean abundance of all units within a management type (C =
control, T = treatment, W = watergrass) during a survey period (6-8 days; 2 surveys per
wetland unit = 6 surveys/point) + standard error.
Seasons are also shown and represent
wetland drawdown dates (1 = all wetlands flooded, 2 = control wetlands drain (and some
watergrass), 3 = treatment and watergrass wetlands drain).
Exception -- 2009 treatment
wetland drained during season 2.

Late-winter through spring use by dabbling ducks and surface-diving birds was not affected by delayed drawdown. However, delayed drawdown may positively affect shorebird and large wading bird populations by providing extended periods of resource availability (food, cover, and roosting sites) during spring migration. Cumulative impacts from multiple years of modified hydrology remain unknown, as well as any impacts during fall or early winter.

This study suggests that 1-2 years of delayed wetland drawdown imposed on a wetland unit would not adversely affect waterbird use during late-winter through spring. Nevertheless, wetlands subjected to delayed hydrology should be rotated to avoid cumulative impacts, or further long-term research should be conducted. A diversity of wetland types should be maintained in the GEA to ensure the greatest diversity of waterbirds is supported.

Revised March 18, 2010