I have an early map of San Francisco (circa 1900) indicating that Lake Merced opened to the Pacific. True? Was it dammed up?

-- Christopher Brian West (chrisw@access1.net), January 15, 1999

Answers

Yes, indeed, it was. It originally had somewhat larger shoreline, and and actual egress to the Pacific Ocean, but was also fed fresh water from several streams, the two main of which decended out the Twin Peaks area some seven thousand feet into the hills. Seemingly, at hight tides salt water intrusions from the ocean occurred, so the lake itself was in a constant state of flux regarding its saline content. I understand that there are still species of fish in Lake Merced which, due to this historical legacy, are salt-and-fresh water adapted.

-- Chris Dichtel (cyberiad@earthlink.net), February 01, 1999.

Lake Merced may have been like other coastal lagoons in the San Francisco area. The lagoon would be dammed up by beach and dune deposits, water levels would rise as inflow exceeded outflow (through groundwater flow at relatively low rates) and then would occasionally burst with exciting results. Youd and Hoose (Historic Ground Failures in Northern California Associated with Earthquakes, 1978, USGS Professional Paper 993) documented several examples near Lake Merced, despite that several of failures had nothing to do with earthquakes. For example, in 1852, "The waters of Lake Mercede, in the vicinity of the city, and which cover several hundred acres, sank about thirty feet. Shorly before midnight of this day, a shock like that of an earthquake was felt by parties residing near this placed; the following mornning it was discovered that a great channel between the lake and the sea had opened, through a broad and high sand bank, during the night, by which the waters had found a way and had been discharged... The most probable conjecture is, that the excessive rains of the season had simply forced open a passage through the broad and loose sand-bank from the lake to the ocean. Formerly the lake had no visible outlet whatever; and its waters had insensibly been kept at about the same level."

People who visit Rodeo Lagoon in the Marin Headlands also note the same phenomena there. Although less dramatic and probably less energetic, the waters occasionally rise to the breaking point and cut a breach through the beach ridge. The sand is MUCH coarser and will be eroded more slowly.

Someone may be able to confirm that the lake level is presently regulated by man. However, along the path these periodic floods took, there is likely a wide swath of loose sand that will be highly liquefiable in the next big quake. Zookeepers beware!

-- Jonathan Pease (Zancat @msn.com), March 12, 1999.

During the research for my book, From "The Top of the Hill"...Growing up in the Daly city of the `20s and `30s, I found in the history of the Spring Valley Water Company the following information, which may or may not be a correct answer to your question.

Lake Merced was fed by an underground spring that produced up to three million gallons of water a day. In fact it produced so much water, a brick-lined cement ditch, some 1,840 feet long was build in 1897 to carry excess surface water to the ocean. I suppose it is possible that the ditch could have been shown on a 1900 map as an opening to the sea.

Bernard C. Winn www.sfchangehappens-books.com berncwinn@hotmail.com

-- Bernard C. Winn (berncwinn@hotmail.com), November 11, 2001.