Both Island Park Reservoir and Grassy Lake are administered by the Forest Service. Many recreation facilities have been constructed at Island Park Reservoir. There is good fishing for rainbow trout and coho and kokanee salmon at the reservoir, and it is used by large numbers of waterfowl during their migrations. Snowmobiling is a very popular activity in this area. Grassy Lake is a small reservoir located just outside the southern boundary of Yellowstone National Park.
Visitation is light but there is some trout fishing.
The geology of the area between Minidoka Dam and Arrowhead Dam is complex. The mountain masses have been extensively folded and faulted. They are made up of crystalline, metamorphic, volcanic, and consolidated sedimentary rock ranging in age from Precambrian to Quaternary. The valleys are structurally depressed areas containing large deposits of volcanic rock, lake sediments, alluvial and fluvioglacial deposits, and windblown silts. The valley deposits have been collecting from early or middle Tertiary time to present.