River-Mouth Gravel

One sample of about 270 kg suffices to represent the petrographic composition of the river-mouth sediment of all fractions < 80 mm. The “petrographic mixing” of the rivers is so perfect that even after larger floods there is no change in composition.Petrographic composition is closely connected with grain size, thus an uncritical comparison of different fractions must yield misleading results.The amount of granitic and metamorphic debris in the rivermouth sediments (16 –80-mm fraction) correlates nicely with that of the effective rock composition of the 19 feeder basins or, in other words, the predicted sediment composition (granite: r = 0.76*, metamorphic rocks: r = 0.71*); but the sedimentary rocks do not (r = 0.45). Only the Jurassic limestones climb to r = 0.87*. The correlation source/sediment basing on the 2– 16-mm fraction is much worse.