Adobe Creek Watershed

Adobe Creek Dam

• An earth filled dam with a maximum height of 23 feet and a length of 1,300 feet (ft)
• Constructed: 1963 to 1965
• Watershed: 6.2 square miles
• Adobe Creek Dam:
  • Height (max): 36 ft
  • Length: 1280 ft
  • Constructed in 1962
  • Watershed (drainage area): 8.0 square miles
• Reservoir:
  • At Primary Spillway: 90 acre-feet storage, 24 acre surface area
  • At Secondary Spillway: 695 acre-feet storage, 64 acre surface area
• Primary Spillway: concrete drop inlet with anti-vortex hood
• Secondary Spillway: concrete weir

Adobe Creek Dam Operation

The Primary Spillway limits outflow to Adobe Creek to 940 cubic feet per second (cfs). When inflow exceeds the outflow, reservoir levels increase. The stored water is detained until inflow reduces to below outflow. This reduces the peak flows downstream and reduces flood risk. While peak flows are reduced, high flows are prolonged. No water is released downstream during the summer months. 2 agricultural interests pump water from the reservoir for irrigation of walnuts and grapes. Normal operation of the reservoir is "hands off" with flows being regulated by the design of the Primary Spillway.

Highland Creek Dam

• An earth filled dam with a maximum height of 61 feet and a length of 510 feet
• Constructed: 1963 to 1965
• Watershed: 12.4 square miles
• Highland Creek Dam:
  • Height (max): 76 ft
  • Length: 510 ft
  • Constructed in 1960
  • Watershed (drainage area): 14.1 square miles
• Reservoir:
  • At Primary Spillway: 1,090 acre-feet storage, 72 acre surface area
  • At Secondary Spillway: 3,500 acre-feet storage, 146 acre surface area
• Primary Spillway: concrete drop inlet with anti-vortex hood
• Secondary Spillway: rock and concrete weir

Highland Creek Dam Operation

The Primary Spillway limits outflow to Highland Creek to 560 cfs. When inflow exceeds the outflow, reservoir levels increase. The stored water is detained until inflow reduces to below outflow. This reduces the peak flows downstream and reduces flood risk. While peak flows are reduced, high flows are prolonged. A small amount of water (< 0.5 cfs) is released downstream during the summer months to recharge nearby water wells. Normal operation of the reservoir is "hands off" with flows being regulated by the design of the Primary Spillway. Summer operation requires periodic adjustment of the drain valve to maintain the summer release, as the valve is subject to blockage with debris at the bottom of the reservoir.

Reservoir District

In 1967, the Highland Springs Park was developed on the south side of Highland Creek Reservoir by the District in cooperation with the California Wildlife Conservation Board. The park provides recreation opportunities and fishing access. Restrooms, picnic tables and barbeques are provided. There is also a series of horseshoe pits and boat launch facility. Gasoline powered boat motors are not permitted on the lake. The park is maintained by a caretaker who is under contract with the District. An adjacent disc golf course and 2 shooting ranges are built and maintained by separate clubs. Numerous non-motorized use trails are also available. The trails are constructed and maintained by volunteers.

Adobe Creek Channel

• Constructed: 1963 to 1969

6.5 miles of channel were "improved" between the Bell Hill Road crossing down to the mouth at Clear Lake. The channel was enlarged and/or straightened to provide this flood capacity. Channel design allowed for growth of vegetation on the banks of the creek. The channel was designed to convey a 10% annual chance flood or 10-year flood, because of the predominantly rural agricultural nature of the area. Less frequent floods, such as the 1% annual chance flood, or 100-year flood, continue to cause flooding in Big Valley.

Fire Roads

Approximately 22 miles of fire road were improved around the perimeter of the Adobe Creek watershed. The roads were to be maintained to allow fire suppression access by the California Department of Forestry and Fire Protection (CalFire). With the change in firefighting methods since the inception of the project 60 years ago, regular maintenance of these roads is no longer necessary. Due to budgetary issues the District no longer maintains these roads.

Issues

Construction of the two dams has nearly eliminated new gravel coming into the Adobe Creek Channel and the channel improvements have increased flow velocities increasing channel erosion. The county also permitted limited instream gravel mining in the 1970s and 1980s. These factors have led to channel downcutting (over 6 feet of downcutting in some areas) and bank failure/erosion, which has reduced groundwater supplies in western Big Valley.

Operation and maintenance (O&M) of the project is funded through District Zone 1, a benefit assessment area established under state law. Zone 1 was formed in 1954 to fund operation and maintenance of the Project. Zone 1 funding is a percentage of the property taxes assessed within the District, as limited by Proposition 13 (1978). Increases from this funding source have not kept up with inflation, fee increases, and labor costs, etc. Annual state permit fees consume over half of the funding. A special benefit assessment to ensure adequate funding for O&M was turned down by the voters in 2005. Due to limits in funding, O&M activities have been reduced to a minimum. Operation of Highland Springs Park is paid separately from the District's general budget.