High on the west slope of Mt. Hood, the Sandy begins as a stream, then winds fifty-six miles northwest to join the Columbia River. Its watershed is 508 square miles. About 75% is forested, and most of the watershed is part of Mt. Hood National Forest.

For its final five miles, the Sandy River widens along the way to its confluence with the Columbia River. The river becomes shallow as it flows through a large delta of sand and other sediment that accumulated over thousands of years. Originally, the Sandy River split in two at its delta. The island between its two channels, at the top end of the delta, is known as Sundial Island.

Currently, almost half of the fifty-six mile long Sandy River is considered wild, scenic, or recreational.