I wandered down Missouri D Road in Madison County and discovered Silver Mines, where the St. Francis River runs over hard rhyolite rocks. Missourians call it a "shut-in."
Between Tiemann Shut-in upstream and the low-water footbridge at Silver Mines the river drops sixty feet over the course of 2.3 miles.
It's a wild ride and kayakers love it. They put in at Tiemann, slalom through the Big Drop, the Cat's Paw, the Double Drop, and the Rickety-Rack; rest in the slow water between Mud Creek and Turkey Creek; negotiate a willow jungle before facing the Silver Mines dam. If the river is up and the eight-foot dam is submerged, they portage around it to avoid the hazardous "pourover." If not, they steer through a breach in the dam and negotiate the the rocks at Silver Mines.
The ride ends in quiet water near a low-water bridge.