Flecks of gold were washed down the Rhine from the Aar region of Switzerland. They were deposited along with quartz, mica and feldspar as sediment in the gravel banks of the river[...] Sifting the sand and gravel for gold was one of the oldest occupations on the Rhine. The Celts had done it in the third century BC. Strabo reported on the river's riches around the time of Christ's birth, and the Romans supposedly shipped so much Rhine gold back to Italy that they depressed the price[...] The new hydrological conditions made it impossible to continue extracting gold from the gravel: the high waters came and went too quickly to leave anything other than tiny and irregular deposits.
David Blackbourn, "The Conquest of Nature", 2006.
Gottfried Tulla's efforts to straighten the meandering Upper Rhine had taken effect by the time Wagner's "Rheingold" opened in 1869. Yearly output of gold had dropped from several kilos a year in the 1840s to a couple of hundred grammes, and the state treasuries of Baden and Bavaria stopped recording the take.
Salmon, for which the Rhine was famous, disappeared too, and eels (which breed in the Sargasso Sea and so were unaffected by the engineering) colonised the river.
Malaria ceased as the river marshes dried out.