The Ins and Outs of Lake Merritt’s Tidal Gates

by Dr. Richard Bailey

Lake Merritt is a tidal estuary controlled by tide gates, operated by Alameda county. The controls are based mainly on flood control, which is why they were built after a nasty Oakland flood in the ’60’s.

The gates are controlled by setting a computer to one of four modes, and have recently (or soon will) have the capacity to be set remotely, rather than having to send someone in person to the station located under the 7th Street Bridge. The normal mode (80 – 90% of the time) is for one or perhaps two sides of the structure to be open to inflow and outflow all the time.

When heavy rains and runoff threaten to occur during high tides, the gates are closed to prevent incoming tides, then opened during low (outgoing) tides to let the water drain. In addition to being able to close gates, there are diesel operated pumps that can pump against incoming tides to prevent flooding. They are rarely used, but needed to prevent flooding. The whole system can handle a 50 or 100 year storm, but not a 500 year storm. A while back, the docks at the Lake Chalet were briefly flooded, so the County needs to stay on top of conditions.

A problem can occur when high incoming tides are kept out during rain events. Fresh water (rain) does not mix with the heavier salt water, and stratification occurs, which causes the bottom, the salty layer, to lose oxygen. This is not common, but if levels reach zero, one, or two, it restricts where living things can function. If it reaches zero top to bottom, everything dies, as occurred in our devastating August fish kill.

The Lake Merritt Institute is developing plans and is raising funds to address this oxygen issue. We need a lake-wide aeration/oxygenation system, which consultants from the contractor LakeTech are now working on. Such a system could prevent illegal oxygen levels while simultaneously maintaining flood control.

Lake Merritt evolved with natural tidal flooding that was greater before the channel was dramatically narrowed by civilization, which reduced overall tidal flow. Recent removal of the bottlenecks at 12th and 10th St. have resulted in increased tidal flow (thank you Measure DD) but still not what it was way back in the beginning. In general, a balance must be maintained between flood control during the rainy season, and water quality.

Recently, Alameda county has not responded to city concerns over tide gate operation, and a consultant report that suggested operating the gates to maintain high lake levels for aesthetic purposes during the day, with drainage only at night. This would be devastating for things living in Lake Merritt. The county has not responded to concerns expressed by the Measure DD coalition about a year ago.

The new monitoring buoys now allow anyone to check the level of the Lake (and thereby the tide and gate closure status) 24/7. Thus, we can now correlate other events to tide flow, which is a major improvement. Now we can keep an eye on them.

The tide gates are controlled by skilled county workers. Gate operation changes could probably not have prevented the recent fish kill (they were likely open and allowed the red tide to come in from the Bay.) However, if we find that overall nutrients in the Lake increase due to incoming tides carrying effluent from the EBMUD discharge (which is not too far away), then it may be possible to fine tune the incoming tides to restrict such flows by selectively closing gates at the right time.

 This would require constant monitoring of channel water for Nitrogen, and chlorophyll, but there is currently no monitoring buoy in the channel. We need one there, but we don’t have the $20k for an annual lease. So the technology to balance water quality and flood control is there; it just needs to be funded.

Editor’s Note: Dr. Bailey, the former Executive Director of the Lake Merritt Institute, submitted this article in response to an inquiry from Sarah Van Roo and it nicely complements the report she wrote for last month’s Splash Pad News about LMI.