The Not Quite Adventures of a Professional Archaeologist and Aspiring Curmudgeon

Thursday, March 12, 2009

Pluvial Lakes

One of my favorite aspects of archaeology is the fact that we can trace the way that the world looked well into the human past. What did San Francisco look like before a city stood there? What was the Midwest before it was expanses of farmland? It gives me a “hair standing up on the back of the neck” thrill when I think about these types of things, and the fact that we can answer them, and that they needn’t remain mysteries makes it all the better. Let me give you an example.

The town where I live, Scotts Valley, was once host to a mountain lake. My apartment is in a location that was once near the lakeshore. The lake persisted in the area until the early Holocene (so, give or take 8,000-10,000 years ago) and was home to some of the earliest humans in California. If you look at the place now, there’s a definite basin, but no sign of a lake other than a creek that runs through the west side of town.

The location probably looked something like this in the past, but without the clapboard house on the ridge (not my photo):

Now it looks like this (photo from the Santa Cruz Metropolitan Transit District):

So, what happened?

Well, a few things. One of the major ones was climate change during the late Pleistocene and early Holocene eras. A combination of increased rainfall and decreased evaporation led to the formation of pluvial lakes in much of western North America. These largely formed in areas where a previously (and subsequently) arid climate resulted in very few water channels, so water tended to flow into basins but not out of them. A few of these lakes have left small remnant lakes behind them – the Great Salt Lake in Utah being one such example – others have left massive dry lake beds (such as China Lake in southern California). Click here to see a map of where the lakes were and how large they were.

I don’t know that the lake in Scotts Valley would have been considered a pluvial lake – Scotts Valley is not an arid zone, and while it is certainly a basin in which a lake did form, it lacks many of the characteristics of the other pluvial lake regions (such as being a large, flat area). Nonetheless, increased rain and decreased evaporation aligned with minimal outflow resulted in a lake. And, when people arrived in California, they were drawn to this lake. A population of people thrived here, making use of the rich resources surrounding the lake.

Until the lake vanished.

Of course, the lake didn’t vanish all at once. Rain decreased, evaporation increased, drainage improved as a creekbed appeared on the south side, and the lake slowly dried up or drained away. As the lake shrank, the settlements surrounding it moved closer and closer towards the remnant, like a shrinking ring, until all that was left was a creek. The smaller lake and eventual creek, obviously, couldn’t support the same number of people for the same amount of time as the large late-Pleistocene/early-Holocene lake did, and so populations made more use of other locations, and dispersed across the landscape. Which is not to say that the creek stopped being used, but rather that it became less important than the lake had been.

But the lake may not remain gone. The basin still exists, and should we find ourselves in a situation where precipitation outstrips evaporation, and the path of Carbonero creek gets somehow blocked, we could have another lake, this time drowning the town of Scotts Valley (or, more likely, the remains of the town – we’re talking about events that probably won’t happen until the far future). This is what is often referred to as an “oscillating lake” a lake that grows, shrinks, sometimes disappears depending on the long-term weather patterns (though, again, in this case, the creek would have to be clogged to prevent drainage of the waters).

A good example of this sort of lake is Lake Cahuilla, in southern California. Now the basin in which the Salton Sea sits (the Salton Sea is an artifical lake created due to a series of water management mistakes – and it really could be the subject of a screwball comedy), the dry lakebed was one of the larger pluvial lakes during the early Holocene. It grew and shrank depending on the amount of rainfall that occurred during any given period of time. When the lake was large, it attracted numerous settlements as people came to make use of the abundant resources that the lake provided, resulting in a large number of people from a large number of cultural and language groups being present around the edge of the lake. As the lake shrank, the people closed in, again forming a shrinking ring that collapsed in around the remnant lake, leading to conflict, strife, and general bad blood as the different groups impinged on each other’s territory and competed for increasingly scarce resources. But the lake was not static, it would sometimes shrink, sometimes grow, and these oscillations would draw people in, and then mix people up and push some of them out, creating a strange and fragmented cultural/linguistic landscape that confused the hell out of early European explorers and settlers, and continued to confuse anthropologists for a good chunk of time.

Isn’t ecology fun?

At any rate, in addition to the weird thrill that consideration of these past landscapes gives me, it’s also instructive in telling us something about how humans react to environmental change. It touches on how a resource such as a lake (which provides not only water, but also wildlife and plants) can create a way of life, and how the loss of that resource can force those reliant on the resource to have to adapt. In the case of Lake Cahuilla, it also demonstrates how the loss of that resource often leads to conflict as competing groups fight for what is left.

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