The southern half of the reservation is open plains and the northern portion resides in the treed Rocky Mountains. Mammals and birds migratory paths cross the reservation seasonally, including mountain lion, black bear, elk, Canada geese and turkey. Rainbow, brown and cutthroat trout are stocked in seven lakes on the reservation, but annual conditions such as low precipitation result in high pH-levels. From 1995 to 2000 the lake levels were severely low due to drought; As a result, most of the fish were killed off during those years. The reservation sits on the San Juan Basin, which is rich in fossil fuels. The basin is the largest producer of oil along the Rocky Mountains and the second largest producer of natural gas in the United States.