A PILOT STUDY ON NEEDLE ICE INDUCED STREAM-BANK EROSION IN THE MASHAI VALLEY, LESOTHO HIGHLANDS

ABSTRACT

This pilot study examines the likely impact of needle ice processes in the bank erosion system along the Mashai stream in eastern Lesotho. The objective is to assess the potential role of cryogenic processes as contributors to stream-bank erosion along the upper reaches of the Mashai stream. Four collection troughs were inserted along the Mashai stream-banks in 1993 so as to compare the rates and volumes of sediment movement during a cold/dry winter and a mild/wet summer season. In addition, individual particles were painted along transects and their displacement recorded on banks of variable gradient. It is found that despite the significance of frost activity during winter, rain wash and stream discharge are more effective in sediment mobilization on the stream-banks than are needle ice processes at the monitoring sites. Nevertheless, it is concluded that needle ice plays an active role in sediment mobilization along the stream-banks throughout much of winter and is an important indirect erosion agent by disaggregating earth materials for easy fluvial entrainment.