Delivery and fate of fluvial water and sediment to the sea: a marine geologist's view of European rivers

Abstract

Despite their relatively small drainage areas, European rivers reflect a wide variety of hydrologic regimes, although with very few exceptions they have been strongly affected by human activity. Scandinavian rivers (particularly those draining Iceland and western Norway) can have high runoff, and, except for those draining Iceland, all have very low suspended and dissolved sediment loads. Northern and western European rivers have somewhat lower runoff, among the lowest suspended sediment yields in the world, and anthropogenically enhanced dissolved solid loads. Annual discharge of many of these rivers appears to vary inversely with the North Atlantic Oscillation index. Rivers discharging from the southern Alps into the Mediterranean Sea have relatively high runoff, high suspended sediment yields (reflecting younger, more easily erodable rocks as well as generally smaller drainage basins), and high dissolved yields, although presumably with somewhat less human influence. European rivers and their estuaries tend to reflect the terrestrial environments of their drainage basins (i.e. climate, landscape geomorphology, geology), but they also display strong anthropogenic signatures. Sediment erosion increased dramatically in the last several millenia in response to deforestation, farming and mining. In the past 50 years, however, increased soil conservation and local reversion of agricultural land to forest, as well as river diversion and dam construction, have decreased the suspended sediment loads of many European rivers. Improved mining and manufacturing techniques, as well as more effective use of fertilizers and improved waste treatment, almost surely will result in lower dissolved solids and nutrient fluxes to the coastal environments, which presently are the highest in the world. The long-range effects of changed land use on estuarine and coastal environments remain to be seen, although decreased sediment loads in the past 20-40 years have already resulted in increased shoreline erosion. Decreased nutrient fluxes almost certainly will affect water quality in European coastal waters, and decreased silicate delivery by some dammed rivers may result in proliferation of new (and perhaps harmful) estuarine and coastal ecosystems. Everything points to further changes as European rivers and their drainage basins continue to change in the coming years.