The materials in this section of the UMRWC website have been extracted from the website of Mariposians for the Environment and Responsibe Government (MERG) a local non-profit that disolved in 2017. It is reproduced here for historical interest. Most of this material has not been updated since 2013/2014, but the page on the spillways at Lake McClure was updated to include a comparision with the situation at Lake Oroville and the dam and spillways there. Navigation to other pages of this section are available at the bottom of each page.

The Spillways at Ovoville and New Exchequer Dams

This page has been updated to included information gathered after the Oroville Dam Crisis.

The design and construction of reservoir spillways was once a fairly wonky subject that likely only interested dam engineers and operators. That all changed after the Oroville Dam Crisis that caused massive evacuations of downstream communities when the gated spillway started to fail, and the emergency spillway started spilling water. As you will read in the next few sections, spillways play a very important role in dam safety in that they provide a way for a reservoir to release water quickly and avoid water flowing over or overtopping the main dam. Earthen or embankment dam, like the Oroville and New Exchequer dam at lake McClure, must never be allowed to overtop as that would lead to a massive dam failure and downstream communities like Snelling or Livingston, in the case of the New Exchequer, would be hit by a massive wall of water within minutes of the failure.

In normal operation, dams release water via an outlet at the base of the dam for use by their water customers and in the process typically generate some of the electricity needed to convey the water around California. However, after particularly heavy rains if the reservoir is approaching capacity, the dam operator may need to release more water than that outlet can accommodate. Hence the use of the gated and emergency spillways.



In the picture above you can see the cental dam and the spillways at Oroville. Just as you would expect, with a gated spillway, the flow rate is regulated by opening and closing a series of gates, and ideally the water passing through those gates will have a well-defined path to follow as it makes its way down to the river below.

The Oroville gated spillway had a very long concrete structure that when used in February 2017, started to fail.

Such a failure can lead to erosion that could potentially also lead to a massive dam failure. When operators at Oroville tried to limit the flow down the gated spillway, the reservoir level then increased to the point where water also started to spill over the emergency spillway. When that area started to erode as well, it led to the emergency evacuation of the downstream communities.

Clearly major water flows rushing down a spillway can cause significant erosion and it is therefore critical that spillways are properly designed and maintained to avoid catastrophic dam failure and loss of lives downstream. After the Oroville Dam Crisis, the State of California started taking a hard look at a number of the other reservoirs in the State, including the New Exchequer and Lake McClure. It is critical that the State continue to be involved in evaluating dam safely going forward.  

In 2012 MERG was invited to tour the New Exchequer dam and some of the following photos were taken at that occasion. Other photos are available online.





As can be seen, the path for the water below the gated spillway at New Exchequer has a much less well-defined path for water to flow when compared to Oroville.  The emergency spillway flows into this same area.

The continuation of the spillway downstream (right) is also poorly defined and crosses several roads until it finally joins Lake McSwain about 1.5 miles down steam from the lake. While we don’t doubt that the water in a 500-year flood will find its way, it is a question of how much damage is done as torrent of water finds its own path. It would seem that MID might want to do a spillway modification project, but perhaps that project should be to improve the existing spillway at the current maximum lake level.





A Google Earth image of the New Exchequer Dam from 2018 suggest that not much has changed.



FERC Project Boundary Question

The Effect on the Limestone Salamander