Sediment sources in the channel head
- a transition zone from the hillslope to fluvial processes (NW Poland)
M AŁGORZATA M AZUREK
Institute of Geoecology and Geoinformation, Adam Mickiewicz University
Dziegielowa 27, 61680 Poznań, Poland, gmazurek@amu.edu.pl
In Pomerania (NW Poland) , channel heads are unique components of the fluvial system where denudational slope system passes into the channel system. In the study area, which covers the southern part of the Parseta catchment (617.2 km 2 ), seepage erosion and groundwater sapping are the primary mechanisms of stream channel initiation.
Groundwater erosion cause headward channel growth and lead to the development of an oval niche or an elongated alcoves with a bottom area ranging from 0.007 to 0.1 ha. In spite of their small areal extent, these headwater zones take significant part in the supply of suspended material to fluvial transport. The objectives of this study are:
1. to determine the dominant erosion processes in headwater zones, and
2. to discuss their role as sediment sources.
Groundwater outflows create conditions favourable to the concentration of water in the sub-slope zone and to the possibilities of carrying material away from the headwater zone. The morphology of channel heads with considerable slope gradients is also shaped by mass-movement. Depending on the mechanical properties of the substratum, slope gradient, and the location and intensity of outflows, headwater areas are shaped by such types of mass movement as: fall, landslide, earth flow, and creep of the water-logged substratum. Erosion due to the uprooting of trees and animal action also contribute to the sediment load. The development of headwater areas and supply of material are especially intensive in the winter season as a result of needle ice and snowmelt action; in summer the degradation of their slopes is controlled by plants.
Depending on the discharge volume of outflows (1 ÷73 l/s), products of slope erosion are transported from the slope system to the fluvial system, and the bottom of the channel head becomes a transport zone. When the activity of flowing water is low or sediment supply from the slopes is large, the water's transport capacity is insufficient to carry the material away, which usually hold-up the development of the headwater area temporarily.
In general, the accumulation of material at the foot of the scarps is very small, indicating effective removal of sediment.
The repeated morphological mapping of the channel heads reveal the group of processes responsible for supplying sediment in the first order streams: the surface and groundwater erosion, mass movement, frost and snow action, and the action of animals.
Observations of sediment transport and runoff in the headwater zones can also improve understanding of channel initiation processes.