Haystack Dam is a zoned earthfill offstream storage facility located about 10 miles south of Madras, Oregon. Most of the stored water in the reservoir is diverted from the North Unit Main Canal which receives its water from Wickiup Reservoir. Releases from Haystack Reservoir flow in a feeder canal back to the North Unit Main Canal for use in the service area. The feeder canal also acts as a spillway.
Because of the distance from Wickiup Reservoir to the lands of the North Unit, the regulatory storage provided by the 5,600 acre-foot Haystack Reservoir is required. The dam is an earthfill structure, 105 feet high at its crest with a width of 25 feet and length of 1,200 feet, containing 535,000 cubic yards of material.
Bedrock at the damsite was mapped as the John Day Formation. This mapping indicates the dam is a member of the formation consisting of rhyolite, ash-flow sheets, bedded, welded and lapilli tuffs with interbedded tuffaceous sedimentary rock. The implied source area for the volcanic material is Juniper Butte; a rhyolite dome with associated flows located two miles southwest of the dam. Robinson infers a north 75-degree trending fault one half mile south of Haystack Dam. The fault displaces John Day Formation and the underlying Clarno Formation but does not cut the younger Madras Formation surrounding the butte and overlapping the older formation. Surficial deposits on the valley floor along the axis of the dam ranged from 0 to 24 feet in depth. These deposits were alluviums, consisting of silty sand, coarse sand, clayey sand and gravel. Toward the left abutment the overburden deposits were alluvium and talus and near the base of the slope about 30 feet thick. Bedrock under the surficial deposits in the valley consisted of rhyolite and tuff and was exposed at the surface near the top of both abutments.