Variation in the chemistry of the North Fork of the Shoshone River following a storm cannot be explained simply by dilution of the base flow by rain water. Factor analysis of the variation indicates that during the early part of the flood cycle, the most important control is solution of readily soluble salts from the soil zone. During the later part of the flood, the dominant control is dilution of the base flow. Other significant controls during both parts of the flood cycle are selective weathering of ferromagnesian minerals and leaching of potassium from biological material. Variation in water chemistry during a storm can be used to reveal information about weathering processes occurring at different depths in the soil and weathered rock zones.