The Pedernales at Johnson City

US 281 crosses the Pedernales River just north of Johnson City. The name is Spanish for flint, given for the chert cobbles in the riverbed, and Texans have said it as Purd-n-alice for long enough that the Spanish pronunciation now sounds wrong locally. The river runs 106 miles from western Gillespie County east to the Colorado at the head of Lake Travis, and it is one of the least regulated rivers of its size in Texas. There is no major dam on the main stem, so it still rises and falls with the weather rather than with a release schedule. Johnson City sits a mile south of the water, on the divide above it, which is why the crossing is here and the town is not. Lyndon Johnson grew up on this river a few miles upstream, and the ranch he later ran the country from still fronts it. Average flow past Johnson City is about eighty cubic feet a second, which is not much water, and for much of the year the river is a chain of clear pools on a wide limestone floor. It does not stay that way. The Pedernales drains a steep, thin-soiled basin off the Edwards Plateau and can rise many feet in an hour. The ledges and potholes at Pedernales Falls, some ten miles downstream, were cut by exactly that behaviour repeated over a long time. Upstream, west of Fredericksburg, the river crosses the edge of the Llano Uplift and carries the evidence back down with it: granite and gneiss gravel eroded from rock more than a billion years old, mixed through a bed that is otherwise Cretaceous limestone. It is the oldest material anywhere in this landscape, moving through the youngest.