VERTICAL GRAIN-SIZE AND ORGANIC-CONTENT TRENDS IN NATURAL LEVEES: FIELD STUDIES IN TWO MODERN FLUVIAL SYSTEMS

Field investigations of natural levees from two modern fluvial systems (upper Columbia River, British Columbia; lower Saskatchewan River at Cumberland Marshes, Saskatchewan) were conducted to better understand levee origin and evolution. A wide diversity of levees is displayed at the two sites. Grain size and total organic content (loss-on-ignition) were analyzed in 190 and 119 sediment samples, respectively, collected from 85 auger and 10 vibracore borings.

Levee deposits, typically composed of fine silt to fine sand, are coarsest near the channel and become finer grained towards the adjacent floodbasin. Variations in vertical textural trends are observed to reflect differences in levee growth history. Coarsening-upward levees may form by (1) channel enlargement and bank migration or (2) increased flow competence accompanying avulsive discharge increases. Fining-upward levees result from progressive channel abandonment. Many levees are texturally heterogeneous but show no consistent vertical changes in grain size. They are interpreted to form under conditions of relatively stable channels.

In relatively youthful levees at the Cumberland Marshes, organic content inversely correlates with grain size. Here, upward decreases in organic content are associated with coarsening-upward grain sizes of relatively fast-growing levees. Vertically inconsistent organic-content trends are associated with heterogeneous grain-size trends which are interpreted to reflect complex flooding histories and relatively stable channels. In the Columbia Valley, organic contents and grain sizes correlate well only in the upper parts of relatively older levees, and upward increases in organic content are associated with more gradual coarsening-upward trends which might reflect relatively slower aggradation.