A detailed mapping of the geology of the watershed was accomplished by David H. McIntyre, Ph.D. candidate at Washington State University, who completed the field mapping during the summer of 1961 to 1963. The geology was mapped in detail partly on 1:20,000 scale black and white aerial photographs and partly on 1:12,000 scale color aerial photographs.

The Reynolds Creek Experimental Watershed lies in an erosionally-modified structural basin surrounded by structural and topographic high areas. Volcanic and sedimentary rocks of late Tertiary age overlie a granitic basement of Cretaceous age. The stratigraphy has been subdivided into 5 sequences including (from youngest to oldest) as follows: granitic rocks, volcanics, welded ash flow tuffs and alluvium, pediment gravels, and landslide deposits.

Granitic Rocks is considered a stock of the Idaho Batholith, and everywhere underlies the younger rocks of the watershed. The outcrops vary in appearance, but are generally characterized by steep-faced, rugged pinnacles and crags. They are easily recognized and distinguished from the other outcrops in aerial photographs. Most everywhere it occurs this granite is deeply weathered, giving way to sandy soils on the footslopes. The age of the granitic rocks is late Cretaceous; they apparently achieved a mature erosion surface before the next overlying sequence was deposited.

Salmon Creek Volcanics is a sequence of rock consisting of olovine basalt, andasite lavas, and pyroclastics, all of Miocene age. This unit rests upon the granitic basement and occurs only in a 3100 hectare area in the northwestern part of the watershed. The Salmon Creek Volcanics are overlain by the rocks of the Reynolds Basin group and are the oldest volcanic rocks on the watershed. The entire sequence of this unit exceeds 1160 meters in thickness, but is so faulted that a more accurate measurement is impossible to make. Outcrops are characterized by mature topography, mostly steep slopes with thin soil veneer and sparse vegetation cover. Massive flows of the more resistant formation form craggy outcrops at various localities.

Reynolds Basin Group, an intertonguing complex of basaltic flows, silicic tuff, diatomite, arkosic sand and gravel and felsitic to glassy flows of latite, is of Miocene to early Pliocene age. The total thickness of the individual units may exceed 1525 meters, but rarely does an outcrop expose thicknesses greater than 305 meters. Rocks of the Reynolds Basin Group overly discordantly tilted and eroded fault-bounded blocks of the Salmon Creek Volcanics and were in turn, deformed and channeled before the Pliocene rhyolite welded ash flow tuffs were deposited. Most of the outcrop areas of this unit form rounded slopes with shallow, rocky soils with very sparse vegetation.

Rhyolitic Welded Ash Flow Tuffs are of Pliocene age and do not exceed 90 meters in thickness in an individual flow. The exposures of this unit are limited to the northeastern corner of the watershed and along the middle east perimeter in the Black Mountain area. In the northeast area of the watershed they form bluffs 20 to 30 meters high, but form subdued, steep slopes in the Black Mountain area.

Basaltic rocks cover approximately 38 percent of the total surface area of the watershed. The remaining rock types occur in a considerably lesser extent. Alluvium covers only 2½ percent and occurs mostly along the narrow flood plains of the valley floor. Formation of the Reynolds Basin was begun before that of the Snake River Plain. The north-south trending valley is the locus of a north-south trending asymmetric syncline. Rocks along the west side of the valley are steeply dipping and are broken by faults both parallel and diagonal to the trend of the valley. The axis of the major anticline follows along the eastern boundary of the basin for the major portion of the watershed. Intense faulting is the dominant response to deformation, especially in the Salmon Creek volcanics. This faulting has resulted in the exposure of several granitic blocks which were originally topographic highs on the older erosion surface before the Salmon Creek volcanics were deposited. A major fault with a maximum measured throw in excess of 305 meters occupies the north central portion of the basin. Reynolds Creek follows the trace of this fault approximately three miles as it flows out of the basin through Reynolds Canyon. Throw on the fault diminishes toward the south end of the basin.

The Reynolds Creek Experimental Watershed is a portion of the Payette Section of the Columbia Plateau physiographic province. Relief within the watershed is highly variable, reflecting a complex geomorphic history. The valley floor is of low relief, composed of dissected terraces, pediments and local flood plains. The topography surrounding the valley floor is steep on all sides. The perimeter of the watershed varies from smooth rolling hills to rugged, high relief cliffs. North trending interior ridges and steep narrow tributary valleys open onto the valley floor.