MULTIPLE INFLUENCES AFFECTING CONTINENTAL SCALE SEDIMENT TRANSPORT RATES

Available scaling models allow for a hypothetical polar river, to be compared to an equally sized river draining similar topography, but as influenced by another climate. Polar climates yield less sediment: 1) melt-induced runoff yield smaller flood waves then surface runoff from falling rain; 2) frozen soils and river beds reduce sediment yield; and 3) summer-time polar rain falls is less intense than from temperate or tropical regions. Glacier cover positively influences sediment yield, but since <1% of the landmass has glacier cover, there is not a strong global influence. Compared to arid, temperate, cold and polar climate regions of the world, Tropical land areas occupy only 19% of the land surface, yet deliver a disproportionate sediment load to the world’s oceans, both particulate and dissolved. Biogeochemical weathering and rainfall intensity both scale positively with basin temperature and along with basin elevation strongly influence sediment yield. Tropical rainforests also mediate regional moisture patterns --- rainfall may double given the feedback between the amount of aerosols released and raindrop nucleation, a process that affects more than 60% of the tropical land surface. Yet for all of their uniqueness, for example the dominance of convective and cyclonic precipitation, Tropical Rivers are just as strongly influenced by drainage area and topography as rivers under the influence of other climates. Large rivers have significant year-round base flow and their inter-annual variability is small. As a river basins area gets smaller, the seasonality of discharge and the discharge coefficient of variation increase. Rock type within a river basin may strongly influence a basin’s sediment yield — softer sedimentary lithologies are highly erodible compared to PreCambrian gneisses and granites. However there are no climatic preferences influencing the world’s rock type distribution, which has more to do with plate tectonics than continental erosion.