Water Level Control and Tidal Flows

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WHITE PAPER ON WATER LEVEL CONTROL AND TIDAL FLOWS
Prepared by The Lake Merritt Institute
Updated - October, 2002

ABSTRACT
Historic and current reduction of tidal flows to Lake Merritt is correlated to reduced water quality and diminished aquatic habitat. Periods when water levels are held constant have been linked to low oxygen levels and hold the potential for preventing successful establishment of proposed wetland areas. Although conditions have improved in the last decade, there is room for further improvement by operational and structural changes at the flood control structure. Operational changes could include daily (every 12 hours) decisions regarding tide gate closure, and making personnel should be available 7 days a week to fine tune the balance between flood control and the health of the lake. A newly designed facility could withdraw water from the bottom of the Lake (thus improving water quality) allow tidal action to the entire channel and could provide increased flood protection.

BACKGROUND
Lake Merritt has been a salt water lagoon with natural tidal flows since the ice ages. During the last century however, the Lake’s water level has been controlled. Tidal flow has been restricted, kept out, kept in, and everything else in between. This manipulation has been accomplished in the name of many good causes, but the results have often been unforeseen. Early controls were structural, including a dam across the inlet, and filling the channel between Lake Merritt and the Oakland Inner Harbor.

The Dam: Control began when the 12th Street dam was completed in 1869. What had been a 600 foot wide opening to the Bay was restricted to the area of a drawbridge, thus reducing tidal flow. A wooden floodgate was installed to maintain a the Lake at a higher level, thus eliminating many of the mud flats and marshes. The volume of water entering and leaving the Lake was greatly restricted.

The Channel: The restriction of water flow continued as the channel to the Bay (once a quarter mile wide in places) was filled and narrowed (Oakland Museum Watershed Map, 1993). Tidal flows that had once been adequate for three masted schooners to enter the Lake were diminished. Now, while San Francisco Bay rises and falls six feet daily, Lake Merritt is restricted to 1 - 2 feet of tidal influence. Its like trying to fill a bathtub with a straw; the channel is simply too small to carry more water. With the dam and narrow channel in place, tidal flows to the Lake are a fraction of their former volume. Next >>

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