Volcanic deposition and river erosion have both shaped parts of Idaho over geological time. is the correct choice. Basalt forms from cooled lava, while river erosion can cut deep gorges such as Hells Canyon.

Rivers connect mountain snowpack, valleys, ecosystems, farms, and communities. Seasonal runoff helps replenish waterways, while reservoirs and diversion structures change how water is available. River management therefore involves tradeoffs among agriculture, energy, habitat, and recreation.