23 Sediment Transport and Deposition in Mountain Rivers

In many ways, sediment transport dynamics in mountain environments differ from those of the lowlands. The high relief and the steep slopes are an obvious feature of mountain areas. The cold climate is another factor controlling runoff and erosional processes, and the distribution of vegetation is also controlled by the prevailing climate. Above the tree line, processes of erosion are more intense than those operating below. However, mountain areas are of varying character and the conditions differ greatly from one region to another and from low to high latitudes. One of the main difficulties when attempting to generalise with respect to processes of erosion and sediment transport in mountain areas is the dominating influence of bedrock geology and the thickness and nature of the overburden. The Scandinavian mountains differ in this respect from the European Alpine region: the rocks are less erodible and lakes are more frequent. Sediment sources are also unevenly distributed across drainage basins, so that only parts of the river course may be alluvial and, as a result, water discharge and sediment transport can be extremely variable. A typical mountain river basin in Scandinavia may thus be composed of a number of local erosion and sedimentation subsystems, and it may not be possible to characterise the entire system from records collected at downstream stations. One way to deal with such problems is to recognise the various types of subsystem and to take the effect of each of them into account when river basins of various regions are compared. The Josted~la River basin has been selected as an example of a mountain river in western Norway (Figure 23.1). Unlike the European Alps, the relief is more subdued, with an extensive plateau at 1200-1500 m above sea level (a.s.1.). A glacial valley, Jostedal, is deeply incised into this surface. Sediment in the Josted~la basin is essentially supplied from two types of sources: subglacial erosion, and erosion of moraines and glaciofluvial deposits. This is material deposited by Pleistocene glaciers and by the glacial advance during the Little Ice Age. The transport system is the watercourse of the gravel-bed River Jostedola. In some reaches it is subject to extensive braiding, and several lakes constitute local sedimentation basins. The river basin (with a catchment area to the fjord of 860 krn2; about 27% of which is glacier-covered) is situated on the northern side of the