Abstract

In 1900 the position of the mouth of the Rhine (Alpenrhein) was artificially changed so that the river flowed into Fussach Bay in the eastern part of Lake Constance. The average water flow from 1931 to 1960 was 223.6 m³/sec; the average suspended load was 349.5 cm³/m³, or 454.1 g/m³. Thus, the river supplies an average annual suspended load of 2.571 million m³ which is mainly deposited in Lake Constance. The amount of bed load is about 40,000 m³ per year. Seasonal deviations from the average are extreme; during the peak of the thaw period in the Alps, water flow can be ten times greater and the amount of suspended load can increase more than twenty times. Through extension of the delta out into Fussach Bay, the area of Lake Constance has decreased by approximately 1.2 km² in 50 years, and the average depth of Fussach Bay has decreased from 17.2 to 4.06 m.

The Rhine Delta is composed primarily of silty sands; clean sands and pebble deposits are extremely rare. The average grain size decreases from top-set beds (silty sands) through fore-set beds (silty sands and silt) to bottom-set beds (silt to clayey silt which grades into silty clay away from the delta). The mineral constituents of the sediments are quartz, feldspars (orthoclase and sodic plagioclase), fragments of carbonate rocks (limestone and dolomite), and micas and clay minerals (kaolinite, illite, ledikite, chlorite). The clay mineral content increases with decreasing grain size. The amount of organic substance present is small, but it also clearly increases with the decreasing grain size. The heavy mineral content is characterized by the association garnet-epidote-staurolite-apatitekyanite.