Thunder Basin National Grassland In Northeastern Wyoming between the Big Horn Mountains & the Black Hills. Thunder Basin National Grassland The Thunder Basin National Grassland, located in northeastern Wyoming, is in the Powder River Basin, between the Big Horn Mountains and the Black Hills. Activities include hiking, hunting, fishing and bird and wildlife viewing. Camping is allowed, but there are no developed campgrounds. The U.S. Forest Service manages the grassland, which is made up of federal, state and privately owned lands. The grassland provides forage for livestock and is also underlain with vast mineral resources. The grassland, together with the Medicine Bow-Routt National Forest, encompasses nearly 3 million acres from the north and eastern borders of Wyoming south to the I-70 corridor that traverses north central Colorado.