Origin and Aims of this essay It was not by choice but by the misfortunes or fortunes of the last war, that I became involved with rivers. In December 1946 I obtained a lecturing post at the then Gordon Memorial College at Khartoum and the Principal of the college brought me to confluence of the two Niles and urged me to 'do something' on the biology of the river. I was very reluctant, my experience was limited to lakes in Poland up to 1939, and I did not know anything about work on rivers. The 'equipment' was a rowing boat, hired, and a 'home made' plankton net. This limited our first...