Mount Diablo State Park: Comprehensive Stock Pond Evaluation and Sedimentation Remediation

Abstract:

This project will evaluate between 15 and 20 stock ponds within Mount Diablo State Park (MDSP).  These ponds were constructed for ranching operations prior to park acquisition, and are experiencing various levels of disrepair.  The project will identify and characterize each pond, it's condition, sustainability, sedimentation source, potential for dam and pond failure as well as habitat value to support species of special concern such as California red-legged frogs.  Specific treatment alternatives will be developed for each pond, and a prioritization of treatments will be developed based on various criteria.  As funding allows, ponds will either be repaired and retained, or removed to restore hydrologic function.

Purpose: A comprehensive stock pond inventory and management plan will be developed by this project. The many old stock ponds in Mount Diablo State Park (MDSP) are in various conditions: Some of the ponds are hydrologically sound with intact earthen dams and minor, non-eroding spillways. Many of the ponds have unstable dams and spillways that are deeply eroding and headcutting. DPR must strike the delicate balance between retention and removal of these features. In locations where the old ponds are supporting sensitive species and are in good condition, they should be retained as essential wetland habitat. In cases where the ponds do not support sensitive species, are adversely impacting the landscape through sedimentation, or are unstable and unsafe, full removal would be the best option. Additionally, some ponds may be historic, which will have to be considered and weighed against other factors during the evaluation process. An example of the purpose and need for this comprehensive evaluation is the Bruce Lee Pond on the north side of Mt. Diablo. Swanson Hydrology and Geomorphology prepared a very brief evaluation of the Bruce Lee Pond in 2001. According to Swanson, this pond is supported by an 18-foot high earthen dam that forms a 3 acre (+1-) stock pond. The pond provides extended seasonal marsh habitat for California red legged frog (CRLF), although recent surveys have not been conducted to validate the presence or absence of CRLF. In recent high floods years (1995, 1997, 1998 and 2006), the earthen spillway of the dam has eroded into a large 300-foot long gully. Further erosion of the gully and headward migration of the 3-foot high headcut into the pond area threatens to reduce pond storage and decrease the seasonal longevity of frog habitat. The pond holds about 30 acre feet when full, and further erosion of the spillway may cause an instantaneous failure and draining of impounded floodwater. This may pose a hazard downstream as it drains into a channel that eventually drains into a residential neighborhood downstream. Although the dam is below the threshold for jurisdiction by the State Office of Dam Safety (less than 25-feet high and less than 50-acre feet storage), the stability of the spillway needs to be evaluated for habitat and safety reasons. Additionally, the ongoing erosion of the spilway is a sediment source, delivering sediment to the seasonal drainage below the dam, which is a tributary to Mt. Diablo Creek. Several options were considered for remediation of dam and alternatives. Removing the dam would alleviate the hazard and avoid costly repairs to the spillway, but the seasonal frog habitat would be reduced. It may be possible to re-contour the area into a series of smaller ponds that are excavated into the stream valley, but further investigation would be necessary to determine costs and feasibility. Stabilizing and repairing the spillway to reduce erosion while retaining the pond is also an option. This kind of evaluation should be conducted, park-wide, to address each old stock pond individually. Without a park-wide assessment of stock ponds, and a detailed evaluation of alternatives to determine the best treatement for each site, erosion will continue to degrade water quality in multiple watersheds, and some ponds may be lost if their dams and spillways fail, damaging downstream habitat and possibly resulating in loss of red-legged frog and other wildlife habitat.

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