The Teton River (left; Clark's "Tanzey"),
and the Marias (right)
Dean Hellinger photo
The mainstem of the Teton is free-flowing. Tiber Dam, 78 miles above this point, has interrupted the main channel of the Marias River since 1956. Every spring, as high water from upland snowmelt scours its valley, the Teton delivers a huge load of silt for dilution in the Marias.
On the Marias, however, soil from the many square miles of river basin above Tiber Dam is dropped in the slack water of the reservoir, Lake Elwell. The outflow from the dam, prevented from overflowing the riverbanks below, remains comparatively clear all the way to the Teton.
In 1805, the degree of similarity between the color of the "right hand fork" – "Maria's River" – and the river they had been travelling convinced most of the Corps that it was the better choice than the left fork. Today, with the clear water of the lower Marias only partly muddied by that of the Teton, the choice would appear somewhat more obvious.