December 23, 2002

• Guadalupe Flood

  I’m tired. Really tired. I just got back from spending some time with my dad in New Braunfels. He’s a geeky old guy, from the time before it was OK to refer to yourself as a geek. He’s a scientist.

  We spent most of yesterday afternoon walking up the bank of the Guadalupe river, under the limestone cliffs. There was a huge flood in the valley last July, so we were looking at how the river had changed.

  One of the obvious things is that there’s a huge sandbar across the river from my parents’ house up there. It didn’t used to be there. A few hundred feet long, and maybe 15 feet wide on average. Where it meets the river, it was arranged by the floodwater into three sort of embankments, forming three little lagoon-like eddies. The explanation being that as the water slowed after leaving the limestone outcrop just upstream, it lost the gravel in a big pile. As the pile grew higher, the top was eroded away again by faster water, which then slowed quickly as it dropped off the downstream side of the pile. Thus the river picked up and rearranged, in sections, pebble by pebble, the sandbar.

  We talked aboug things like that. Like, for instance, the limestone I mention above has these deep channels in it. It’s almost like a chunk of an LP record, grown giant. We saw where some of these channels had been made wider, and some had filled up. One in particular, that used to be really deep and had swift water flowing through it, is now dammed by the aforementioned sandbar.

  We took turns noticing things about it. Dad says something like, ‘You can see where the water would carry the sediment through this channel, and drop it where the channel narrows.’ I’d say, ‘And the current acts as a filter for the size of the stones. See how the upstream rocks are larger than the downstream ones? They filter this way all the way down to the sandbar.’ And my dad points to the first stone that had been dropped in the channel, which is about the size of a football and says, ‘Upstream from here is erosion, downstream is deposition.’

  Two nerdy geeks in nature. We did this all afternoon. Mostly my dad talked about dolemitized limestone and sedimentary uplaps. He’s a geophysicist.

  Later we went up to a place I hadn’t seen in over a decade: Devil’s Playground rapids. It’s a really pretty rapid, with two main lines of whitewater formed by cleaved shelves of limestone.

  When I was a little kid, the whole family would be floating through that rapid, in three or four boats. I especially liked the trail that led up to the top of the rapid from the bottom. You could just hike back and drop yourself in, wearing a life-vest. Have a fun float down again and again.

  Mostly I stayed on the banks, though, looking at the exposed tree roots, noticing the way the light came through the cedar leaves, and wondering why no one made tennis shoes that were comfortable when your feet were wet. An infinitely friendly place.

  The far bank, the one inside the curve of the river, was much more mysterious. (You can see it on the left of the picture from the link above.) No one ever went there. It didn’t have the convenient landing spot on the limestone bed, and it was opposite the main road and attendant development that follows the river. An unvisited land not far away. Choked with trees and low vegitation.

  That was all before the flood. After the flood, the rapids and the trail remain, though many of the roots are now more exposed than is entirely safe. But the far bank is now a huge gravel bar. All vegetation removed except the hardiest of trees. And not just the bank, but a quarter mile or so back.

  The river, you see, forms the right side of a sort of T-intersection. The left part comes down from a smaller creek. The rapids are just down from this intersection.

  The energy of the flooded river, however, missed the turn. It slammed into the bottom part of the creek’s channel, tearing away half a hillside. Since the water level was so high, that half-hillside was spread evenly across the whole of the left downstream side of the T-intersection.

  It was stunning to see the change. Where before had been green mystery, now was the surface of the moon.

  • 2:04 am
  • 3 Comments