Pervasive mottling, iron-oxide nodules, and slickensides indicate that alluvial paleosols of the lower Eocene Willwood Formation in the Elk Creek area of the Bighorn Basin, Wyoming were hydromorphic. Two basic groups of hydromorphic paleosols are present: cumulative, which show evidence for concurrent deposition and pedogenesis, and simple (noncumulative), which show evidence for a single episode of deposition and pedogenesis. Lithologic packages consisting of vertically stacked simple paleosols and crevasse-splay sandstones alternate with cumulative paleosols. The simple paleosols are weakly developed, indicating rapid deposition with relatively little time for pedogenesis. Cumulative paleosols show greater pedogenic development, indicating significantly longer periods of pedogenic modification and slower, more episodic sediment accumulation. Differences between the simple and cumulative paleosols reflect differences in the deposition of their parent materials. The packages of simple paleosols are similar to avulsion-belt deposits described from the Saskatchewan River, and suggest that avulsion was significant in floodplain construction. The upper parts of cumulative paleosols were formed on true overbank deposits, which accumulated gradually after avulsion and establishment of a new channel. The lower parts of many cumulative paleosols resulted from a prior episode of avulsion deposition and simple soil formation. Because hydromorphy is associated with texture, these different depositional processes influenced lateral variations in hydromorphy of the cumulative paleosols. Paleosols in the Elk Creek area provide a simple means of identifying avulsions. Sediment accumulation rates suggest that avulsions occurred with a regular period of about 20,000 years and may have been induced by climatic cycles.