Canyon-filling lavas and lava dams on the Boise River, Idaho, and their significance for evaluating downcutting during the last 2 million years

Basalts that periodically dammed the Boise River and its South Fork over the last 2 million years reveal the canyon history and illustrate how lava interacted with impounded river water. Intracanyon basalt flows record a granite canyon successively filled by lava and then recut at least five times in the last 2 million years. The most voluminous flow, Steamboat Rock Basalt, reached 60 kilometers downstream and spread out on the Snake River Plain just east of Boise. Lavas that reached the river were erupted from vents bordering the main canyon and adjacent tributaries. The river canyon was periodically flooded by basalt from these eruptions, reentrenched to a new lower level, then flooded again. This succession resulted in terraces of older flows high above the river and of younger flows lower on the canyon walls. The canyon-filling flows dammed the river and created deltas of pillow basalt and hyaloclastite overlain by massive subaerial basalt. Foreset beds in the hyaloclastite deposits and inclined pillows indicate flow into reservoirs behind the lava dams. A well-documented example of a lava dam is the one formed by the Smith Prairie Basalt, which is about 0.2 million years old. Potassium-argon ages calibrate a canyon history in which the river was lowered at a rate between 0.005 and 0.01 centimeter a year over the last 2 million years. The lava dams interrupted this lowering but were each rapidly incised in about a quarter of a million years at rates averaging 0.03 to 0.07 centimeter a year.