Thursday, April 17, 2008

Rivers to nowhere

When I was a little boy, my outdoorsy uncle told me that, should I ever get lost east of the Mississippi, all I had to do was walk down hill. Eventually I would find a stream, and following that stream would lead me to a road crossing (or, in very rare cases, the ocean).

Needless to say, this algorithm is of little use in central Australia.

The following picture shows a series of closed drainages in southern NT that I flew over on the way home from my last trip.

These drainages, like many small rivers and creeks in central Australia, terminate in a dune field. Some of the larger rivers may actually reach a playa or salt pan, and most of the area of Australia is closed drainages. The following picture from the Stirling ranges in WA shows that even quasi-coastal mountains (45km inland) have streams which terminate in salt flats.

I’m still not sure how working out here will change my views on sedimentation and hydrology. But I do now think of flows as a series of discrete events, rather than a continuous process. While these viewpoint seem odd when applied to permanent rivers, as long as sediment transport is governed by short to medium period flood events, it might be worth hanging on to.

Similarly, I find myself assuming that weathering must eventually result in downstream formation of calcrete/silcrete/ferricrete, depending on surface and ground water composition. I still need to wrap my head around the idea of water just shuffling minerals between pedogenic phases of a closed system, instead of washing stuff away into a vast ocean.


Silver Fox said...

That reminds me of our Great Basin, although we've had some large Pleistocene lakes in the past - Lahontan and Bonneville. And for some reason, a lot of the calcrete I know of is down south in the Mojave Desert, CA, which is technically or mostly open to the sea via the Colorado River - though there are also some closed basins. (Like Death Valley. And the whole Mojave River flows inward and mostly underground). This site says it's an 'upside down and backwards' river.

Following a river in the desert(s) will get you somewhere, but who knows where and whether the water, if present, will be potable.

EcoGeoFemme said...

"Following a river in the desert(s) will get you somewhere, but who knows where and whether the water, if present, will be potable."

But it will probably have interesting stable isotope ratios. :)

Ben said...

For what it's worth, the bushwalker search and rescue folks (in Victoria at least) would certainly not recommend you walk downhill to get yourself found. Standard search strategy is to send a team up the main gully and teams down each feeding gully, and maybe some spurs. This works on the assumption that the lost person walked downhill because it's easy, and got stuck in a scrubby gully. People who walk uphill and look around tend to walk out.