The streams in the subbasin typically erode their channels along less resistant joints or faults in the rock. The streams appear to incise their channels into these weaker zones and create narrow valleys with very steep valley walls. The Selway River channel is relatively straight and has not developed a wide flood plain. Its tributary streams often enter the river as cascades and waterfalls descending the steep valley walls suggesting that the river is exploiting a weaker zone and eroding downward at a faster rate than the smaller tributary streams are able. Numerous geothermal springs in the subbasin suggest a connection to hot rock below.
Climatic extremes occur in the subbasin.
The northern, eastern, and southern subbasin boundaries generally have elevations of 6,000 feet or more and snow-dominated weather typical of the northern Rocky Mountains. Average air temperatures are below freezing in winter and approach ninety degrees in the summer months. Annual precipitation is close to forty inches with more than fourteen feet of snow in the head of the subbasin. The western end of the subbasin receives relatively warm and moist Pacific maritime weather. When the warm, moist air from the west meets the cold air of the higher mountains, intense storms can develop.