Introduction

Abstract

Lake Dillon of Summit County, Colorado, is an impoundment of the Blue River just below its confluence with the Snake River and Tenmile Creek. The watershed drains elevations between lake level at about 2750 m and the mountainous headwaters of the three inflowing rivers at elevations as high as 4300 m. For at least the first decade after its creation in 1963, Lake Dillon was considered unequivocally oligotrophic, as shown by its high transparency. Because of its location, Lake Dillon would be expected to remain oligotrophic indefinitely if the watershed were uninhabited or very sparsely inhabited. Watersheds at high elevations in the Central Rockies are seldom rich in phosphorus because the parent material, which lies relatively close to the surface, consists mostly of hard crystalline rock that is resistant to weathering and poor in phosphorus. In addition, the natural vegetative cover effectively holds the particulate phosphorus inventory, as shown by the clarity of streams in undisturbed areas, even at times of peak runoff.