USING SOIL GEOMORPHOLOGY TO DECIPHER THE FLOOD HISTORY OF RIVERS IN THE WESTERN UNITED STATES

The hazard posed by floods on rivers is an ever increasing problem for human society. In much the same way the relative age of the landscape can be evaluated using pedology-based geomorphic studies, the flood history of rivers can be deciphered using soil development properties formed on the flood plain and adjacent stream terraces. However, to understand the current hazard requires an understanding of the Holocene river history. It is widely understood that soil development on Holocene alluvial soils can be weak, and the failure to take into consideration the effects of stratification that can resemble soil development can lead to misinterpretations. A clear understanding of soil geomorphology and sedimentology is vitally important to making these interpretations.

In the context of the overall physiographic setting, geometric characteristics of a site, attention to the sedimentological properties, and historical flood-producing mechanisms, specific soil development traits can be used to better understand the flood history. For example, the formation of over-thickened A horizons or mollic epipedons can be indicative of frequent inundation and deposition, but at rates less than or equivalent to the rate of soil formation. Conversely, a weak or non-existent A horizon relative to the extent of development of underlying horizons can be indicative of the partial erosion of the ground surface or truncation of a profile caused by a large magnitude flood. Dramatic changes in the parent material can be indicative of cyclic patterns of deposition, changes in the mode of deposition or sediment transport from shallow inundation in a vertically accreting or overbank setting to the higher energy environment of the main channel. Buried soils can be indicative of the avulsion or the lateral movement of the river, a system-wide change resulting in aggradation over an older stable landscape, or a large flood. The clear development of soil horizons, the eluvial-illuvial relationship, indicates that the period of landscape stability has been long enough for pedogenesis to occur. This period of stability is a direct measure of the time that inundation has not been extensive enough to cause erosion or deposition. This provides the basis for the idea of a non-exceedance bound or an upper limit to the magnitude of flooding.