Material and Site Controls of Stream Bank Vegetation

ABSTRACT THIS article describes three specific conditions which limit the estabhshment of permanent vegetative cover for many stream banks in northern Mississippi. Critical conditions are imposed either directly or indirectly by the properties and failure mechanisms of the bank materials. These materials are primarily outcrops of valley-fill units that were deposited during the Holocene Epoch. Material properties and failure mechanisms are relatively consistent within units but differ between units. Two of these specific conditions involve bank height and angle. For a given material, these site conditions indirectly hmit vegetation by controlling mass failure frequencies. Bank instabiUty occurred when bank heights and/or angles were greater than critical, resulting in a loss of vegetative cover. The third condition was imposed by distinctive properties of the early-Holocene massive silt unit. This material is dense and has a well-developed polygonal structure that causes near-vertical bank angles. Outcrops of this material in the channel bank are typically barren of vegetation, but many such outcrops have been relatively stable except where channel entrenchment has exposed excessively weak toe materials. These results establish that bed and bank-toe stability is a prerequisite to the development of permanent vegetation on channel banks for this study area in northern Mississippi. Additionally, these results illustrate the usefulness of classifying channel bed and bank materials as units of the valley-fill system.