Bass Lake Level Reduction

Boat Ramp Access During Low Winter Bass Lake Water Levels

To complete the seismic retrofit at Crane Valley Dam, certain construction activities require Bass Lake to be drawn down to below normal winter levels. PG&E is committed to raise Bass Lake water level during the summer months to 10 feet below normal reservoir levels to maintain summer recreation activities at Bass Lake. Consequently, PG&E has a limited window of time during the winter and spring months to lower the lake level for seismic retrofit dam construction before water levels are raised in the summer months.

Currently, Bass Lake water level is below normal winter levels allowing for construction activity and balancing anticipated rain and weather impacts on dam construction and hydro power operations. During this time of reduced lake levels, unfortunately boat ramps are out of the water part of the time.

In order to facilitate boat ramp access, PG&E ordered extension ramps and submitted a permit application to extend one boat ramp at Bass Lake ensuring boat ramp use during the winter construction activities. Upon approval of the permit and arrival of the ramp extensions, PG&E will work quickly to install the ramp extensions and notify the public when boat ramp access is restored.

In the meantime, PG&E will continue to perform critical seismic retrofit construction to reduce the risk of downstream flooding in the North Fork area in the event of a dam breach during a seismic event.

Bass Lake Level Reduction Reduces Flood Risk

Until the retrofit construction of the Crane Valley Seismic Retrofit Project is complete in 2012, the water level at Bass Lake will be reduced and maintained no higher than 10 feet below maximum water level during recreation season to reduce the risk of downstream flooding in the North Fork area in the event of a dam breach during a seismic event.

As an operating reservoir, Bass Lake water level fluctuates throughout the year and during the winter months is often lower than the level 10 feet below maximum lake level restriction. Hydro generation will continue throughout the construction project.


Risk Analysis

PG&E performed a detailed risk assessment to establish the annual risk of a dam breach at various lake levels. The analysis showed lowering the lake water level to 10 feet under the maximum water level can reduce most of the chance of uncontrolled release. The study also showed further reduction of the lake level to 15 feet only provided marginal benefit.

Based on this analysis, the Federal Energy Regulatory Commission (FERC) and  The Department of Water Resources (DWR), Division of Safety of Dams (DSOD) agreed with PG&E to only require the lake level be reduced to 10 feet from maximum water level which will eliminate most of the risk while recognizing the importance of the lake level for recreation during the summer.

Analysis done by PG&E consultants concluded that the Crane Valley Dam is susceptible to liquefaction and slumping during either a major earthquake magnitude 8.0 on the San Andreas Fault, or a less frequent magnitude 6.25 local seismic event. The analysis showed the west dam as the most vulnerable part of the Crane Valley Dam. Significant slumping due to an earthquake at the west dam could very likely cause water to breach the dam causing downstream flooding to the North Fork community.


Schedule for Lowering Lake Level

The water level was reduced in the spring of 2009 and is now below the 10 foot maximum level.


Impact on Recreation

During parts of construction it is expected that an area by the dam will be closed. However, this work is scheduled to be completed during normal drawdown of the reservoir and should not have an impact to recreation in the summer months.

Recreation will remain open at Bass Lake during the dam retrofit construction and the reservoir will provide plenty of opportunity for summertime uses. We all understand recreation is important to the Bass Lake community, the county, and state.


Impact on Bass Lake Homeowners & Resorts

PG&E and the US Forest Service, in cooperation with the Sheriff, will work with lake resorts and homeowners to allow boat docks to extend further into the reservoir or replace docks where feasible and safe.

Bass Lake Homeowners and resorts are urged to contact Dan Clark, Land Planner at 559-263-7385 to coordinate approval process.