Agate Dam is a zoned earthfill embankment about 13 miles northeast of Medford, Oregon, on Dry Creek. The structure has a height of 86 feet and a crest length of 3,800 feet and contains 421,000 cubic yards of material. The reservoir behind the dam has a total capacity of 4,800 acre-feet (active 4,780 acre-feet).
Agate Dam is in a narrow valley with steep slide slopes. The left abutment juts out into the valley and at a height of 50 feet above the stream flattens off into a bench. Ice or glacial deposits, causing ice or glacial melt water to discharge, once blocked the stream channel over a bench a few hundred feet behind the left abutment. A channel developed which extends from this rock bench to a point in the present channel a few hundred feet downstream from the dam site. Bedrock at the lowest point of the old channel is about 15 feet above the reservoir flow line. Overburden at the flow line on the right abutment consisted of 28 feet of gravel with excess sand and silt. Boulders were hard, fresh andesite 2 to 3 feet in diameter. Much of the over burden between flow line and the valley flow was undoubtedly quite permeable.