Harkins Slough and Expansion of Urban Land Use at Watsonville

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A watershed-related issue examined by the ENVS 560/L Watershed Systems class at CSUMB.


The city of Watsonville, California has approved a measure to annex the area near Buena Vista Drive for expansion. The plan is to develop 2,200 homes with associated schools and commercial buildings. This area drains into the Harkins Slough watershed [1], which is part of the Watsonville Slough complex. Harkins Slough is currently one of the least impacted in that complex from urban and agricultural development. Impacts from development could alter habitat and hydrology in the watershed by increasing impervious surface and urban runoff while decreasing habitat connectivity.


Harkins Slough is slightly north of the city of Watsonville [2] in the Monterey Bay region of central California. The slough extends along Larkin Valley Rd. and the areas around Buena Vista Drive flow into the waterway. It is the longest waterway in the Watsonville Sloughs system, which is a network of primarily seasonal freshwater sloughs that form Santa Cruz County’s largest remaining wetlands [3]. Watsonville is comprised of three sub-watersheds, Watsonville Wetlands(sloughs), Corralitos/Salsipuedes Creek and Pajaro River [4]. The Watsonville Slough system drains a 13,000 acre area and has 800 acres of seasonal wetlands, emergent marsh, salt marsh, and riparian communities (cca). The other nearby sloughs are Watsonville, Gallighan, Hanson, East Struve, and West Struve. Soils around the wetlands are principally clay and silty clay and then farther away are a mix of loams, clay and Tierra Watsonville complex [5] . The average annual rainfall is 22.41 inches [6].

Resource/s at stake

Resources affected by the plan are wildlife habitat and connectivity, flood protection, groundwater recharge, water quality and supply, and loss of rural land use. Current problems in the area include erosion from land clearing and grazing, leaky septic systems, excess sediment and nutrient runoff, subsidence of peat soils, and invasive species [7]. The following link shows the proximity of some other sloughs in the area to development (http://www.watsonvilleslough.org/trailmap1.pdf)


Stakeholders involved in the issue are current residents in the area, residents of Watsonville, city and county government and agencies, Watsonville Wetlands Watch, Ventana chapter of Sierra Club, Watsonville Pilot Association, Land Trust of Santa Cruz County, Coastal Watershed Council,Community Action Board of Santa Cruz County,Open Space Alliance,Santa Cruz County Farm Bureau,Watsonville Wetlands Watch,American Farmland Trust,Association of Monterey Bay Area Governments (AMBAG),Friends of Pajaro Dunes,Trust for Public Land,Community Alliance with Family Farmers, and Friends of Buena Vista (FOBV)[8]. Agencies involved in planning are County of Santa Cruz, City of Watsonville, California Department of Fish and Game, California State Coastal Conservancy, U.S. Department of Interior Fish and Wildlife Service, U.S. Department of Agriculture Natural Resources Conservation Service, U.S. Environmental Protection Agency, and Santa Cruz County Resource Conservation District [9].

Laws, policies, & regulations

The local Sierra Club chapter currently has a case in court to halt implementation of Measure U until a complete Environmental Impact Report (EIR) is completed [10]. While the residents of Watsonville were able to vote on the plan residents of the Buena Vista area were not, as it is still unincorporated by the city.


Many biophysical systems in the area could change with urban development such as soil and air temperature, infiltration, time of hydroperiod for habitat, and type of pollutants in runoff. Harkins slough contains open water habitat over nine feet deep as opposed to the other sloughs which are commonly only one to three feet [11]. Dissolved oxygen in the open water become well mixed in water column and support greater fish and amphibian habitat while shallower water is not mixed and supports algae blooms but not more complex habitat[12]. This feature could also be affected by change in land use.


A study of the effect on hydroperiod and associated habitat from the change in land use and type of runoff would be helpful. An environmental impact report has been requested and would address many of these effects. As surface water in this slough is pumped into a groundwater recharge facility the change in pollutant load and possibility of groundwater contamination linkage could be studied as well. An interesting note here is if urban area creates higher surface runoff there will be more water available to pump out of Harkins Slough and into the recharge facility which is trying to slow saltwater intrusion. See Salinas Valley Seawater Intrusion for more on this topic.

Scientific Tools

Modeling could be done in terms of land use change effect on the hydrograph and flood prevention, seasonal timing of wetland habitat, and groundwater recharge.

Future research

  • The initiation of monitoring with gages to model current hydrograph and water quality tests would fill current knowledge gaps and be the start of a 'before and after' study of the system. This could be a CWSP Master thesis but the timescale is far too long as construction will take years. However, after initial monitoring of the watershed, a thesis could estimate future changes from land use change by modeling.
  • A CWSP Master thesis could be to study and recommend methodologies for minimizing urban runoff, infiltrating runoff from housing more normally into ground water and pretreating runoff before it enters the Slough. The lessened impact achieved by following proposed guidelines could be modeled or quantitative measures could be approximated that could be a useful planning tool. The study and recommendations could be shared with the City of Watsonville and they could use it for upgrading their planning and development guidelines for the subdivision.


1. Watsonville Wetlands Watch. [date unknown]. [Watsonville Wetlands Watch Homepage]. [cited 01/23/09]. Available from: http://watsonvillewetlandswatch.org

2. Conservation Issues of the Vantana Chapter[date unknown]. [cited 01/23/09]. Available from: http://ventana.sierraclub.org/conservation/watannex/sprawl.shtml

3. Swanson Hydrology and Geomorphology, Biotic Resources Group, Dana Bland & Associates, Hagar Environmental Sciences, VB Agricultural Services. 2003. Watsonville Sloughs Watershed Conservation and Enhancement Plan. Prepared for County of Santa Cruz Planning Department. Available from: http://www.fitzwerc.org/downloads/WSCEP_1.0.pdf

4. City of Watsonville Stormwater Management Plan. Available from: http://www.watsonvilleutilities.org/images/pdf/cowswmpmain.pdf



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