4) the water previously in the lake is not the result of precipitation, but is actually a remnant of some previous glacier. It has been steadily draining, slowed by precipitation. Now, it’s done. (looking at the wikipedia picture, I don’t see a lot of elevation above it, so it seems like an unstable place for a lake)

5) The lake outflows through Tenaya creek. The flow through the creek could have been increased, either by some natural process or human action.

6) The weather could have made it a lot more dry up there, or extra solar insulation could have helped evaporate the water, with no precipitation change.

I looked for another explanation.

Glacial lakes are formed by glaciers gouging a depression in a valley and/or depositing a morraine. Once a glacial lake forms it can’t get any deeper and will slowly get shallower. For it to get deeper there has to be geomorphological or geological reason. I can think of 3 possibilities.

1. The underlying rock is permeable and water percolates through the underlying rock allowing the lake to drain underground at times of low precipitation and the lake only fills at times of high precipitation. The rocks in this area are granite so this possibility is eliminated.

2. Avalanches happened at exactly the right location to form the lake. That is, it isn’t a glacial lake at all. It just looks like one. This is possible i suppose, but doesn’t strike me as a reasonable possibility. The USGS thinks this is a glacial lake.

3. The lake has deepened because of tectonic activity. Some lakes are formed by tectonic activity, but it would be an amazing coincidence if it happened at exactly the right location to deepen a glacial lake. Or again it’s a lake that looks glacial but isn’t. Even more unlikely than 2.

So I am left with a dramatic reduction in precipitation as the only explanation. It would be an interesting project to use current outflow from the lake, evaporation estimates and local precipitation data to calculate how low precipitation would have to fall to dry up the lake.

Per http://www.desware.net/desa9.aspx
Present cost for desalinating seawater averages US$1.00/cu. meter, or roughly 0.4c./gal. New plants about half that, and better techniques might halve that again, or say 0.1 – 0.2 c./gal. I’m assuming these costs include amortizing the plant.

Compare this to where I live (Eldorado, NM, near Santa Fe). We pay about 1c./gal, which translates to a $50-75/mo. water bill with a bit of outside gardening. So the economic impact of switching urban California to desalinated seawater should be pretty minimal. Aside from raising the capital to build the plants, especially if it’s an emergency program.

Does seem like California needs to do some contingency planning. Central Valley agriculture would basically vanish for a MWP-style drought.

Cheers — Pete Tillman

W.R., Earle:

Having spent a lot of time on Fallen Leaf Lake, I’m hard pressed to see how there could have been major enough slides to significantly affect the water level. Very little of the perimeter of the lake (less than 10%) is slide-susceptible at all. Really only directly under Cathedral Peak could you get a major slide. Even there, in a 1-by-3-mile lake, I’m having a lot of trouble getting past a few-foot rise (which might be temporary, as it would permit increased evaporation from increased surface area) from a major slide, as might be triggered by an earthquake.

Having spent a lot of time on Fallen Leaf Lake, I’m hard pressed to see how there could have been major enough slides to significantly affect the water level. Very little of the perimeter of the lake (