Watsonville Slough Hydro-Study

Last modified on February 14, 2018.

Final Report (3.8 MBs)

Background

The Watsonville Slough system and its associated watershed extend from the City of Watsonville to the mouth of the Pajaro River. The project’s study area comprises generally the lower to middle portion of the slough system, from approximately Shell Road east to Highway 1 and includes Watsonville Slough, Harkins Slough, Hanson Slough, and the West Branch of Struve Slough. The proposed study will benefit habitat improvement, flood management, water quality improvement and water supply.

The Watsonville Sloughs are a highly valued and unique freshwater wetland resource on the Central Coast. The Slough wetland complex has been modified significantly over the last 100 years, both in size and function. Agriculture and urban uses have encroached on wetland boundaries, portions of the system have been drained to allow farming, and urban development encircles the upper watersheds of three principle sloughs in the six slough system. There are significant draws of deep groundwater to support these activities and there are subsurface drainage structures that discharge shallow groundwater back to the sloughs. Many hydrologic control structures have been installed on surface waters throughout the watershed, including pumps, gates, culverts, bridges, and road crossings. Many of these structures modify the rate at which water flows through various portions of the system, dewatering habitat in some areas while contributing flooding in others. In addition to these control structures and extensive upstream/upland development, recent conversion of highly erodible rangelands to strawberry production has led to further modifications of the hydrologic system with elevated erosion rates resulting in deposition of fine sediments into the sloughs and drainage systems.

Prime farmland and infrastructure have been lost due to chronic flooding of lowland areas within the Sloughs. In addition to the acres of farmland permanently or seasonally inundated by chronic flooding, an innovative water supply project focused on capturing freshwater from the Harkins Slough and using it to recharge the depleted coastal aquifer no longer functions as designed due excessive plant growth and excessive rates of sedimentation.

Project Goals

This hydrologic study will provide essential information to develop and implement strategies to improve management of the Watsonville Sloughs wetlands ecosystem. Proposed projects have been repeatedly delayed due to the lack of critical information on the hydrologic functioning of this complex system. The over-arching goal of this project is to conduct a hydrologic assessment of surface and shallow groundwater flows in the sloughs to support the development of innovative and implementable resource management strategies for the Watsonville Sloughs. These strategies are likely to include enhancement of water supply, flood management, ecosystem restoration, water quality, and recreational opportunities.

In addition to the over arching goal stated above, the following objectives/questions should guide the project and the technical consulting teams approach:

Provide data to support restoration and conservation planning as well as permitting of restoration projects.
Better understand hydrologic status (is the current condition stable or in flux, etc) and trends (is it getting wetter and where) in the sloughs.
Develop and calibrate a hydrologic model that will provide a water budget for the sloughs as well as allow us to simulate and project future conditions.
How can the system be better managed for water supply and recharge?
How will climate change and sea level rise affect the water balance and habitat trends?
Determine whether land is subsiding in sloughs and if so, by how much?
How is water moving through the sloughs and what opportunities does that provide to enhance habitat, water supply and quality, and flood management?
How, where and when are shallow groundwater and surface water connected?