Friday, October 20, 2017

Can't See the Forest for the Creek; Or is it the Other Way Around? California's Mega-droughts

The eastern Sierra Nevada is home to one of the strangest forests I know of. It's not the species of tree that is odd; they are mostly Ponderosa, a pretty but unremarkable tree which can also be found on the adjacent slopes. What's strange is that these trees are dead. Not the "recent forest fire" kind of dead. It's that they have been dead for the nearly a thousand years. Oh, and they are still standing, and are located in the middle of a good sized river.

The West Walker River drains a rather large region in the vicinity of Leavitt Meadows and Sonora Pass. Several forks come together near the junction of Highways 108 and 395, and the river plunges into a narrow gorge before spreading out into the Antelope Valley near Topaz Lake on the Nevada border. The trees, and there are several dozen, are found at the head of the gorge near the campground at Chris Flat. Studies have found that these trees sprouted, grew to maturity, and died within a relatively limited period of time between about 800 and 1350 C.E. (common era, equivalent to A.D.).

The weird part is that the trees pretty much filled the entire canyon bottom. They don't do well immersed in water, which seems to suggest that there was almost no room for a river channel when these trees were growing. This implies that the river was much smaller and therefore there were two crippling droughts that lasted on the order of 200 years  and 140 years respectively. Once the droughts ended, the river filled the channel with sediment again, perhaps supplemented by outwash gravels from the Matthes glaciation (the Little Ice Age).

These ghostly forests exist in other parts of the Sierra Nevada. Tenaya Lake, Lake Tahoe, and Fallen Leaf Lake all have submerged forests that grew to maturity during these periods when the lakes didn't have enough water to flow through their outlets. Oral histories of California's Native Americans also hint at terrible droughts.

In human time frames, droughts are a fact of life here in California. We had extended droughts from 1928-1934, 1960-61, 1976-77, 1988-92, and most notably, from 2011-2016.  Our population has grown so large that each drought becomes more problematic, and we muddle through on the strength of a few extraordinary precipitation years like 2017 that fill reservoirs for a time. What would we do if another century-long drought were to come? And what role will anthropogenic global warming play? I might not be here to find out, but my grandchildren might have a real struggle.