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 exploratory steps to the immediate vicinity of Khartoum. In both the White and Blue Nile we discovered the presence of a pure plankton. This was contrary to opinions expressed in the limited scientific literature available at Khartoum which stimulated our doubts and the search for the origin of this phenomenon. And so, early in our work, we became aware of the longitudinal sequence of events in running water, a fundamental feature of river ecology. of work were daunting; In the Nile, as in other long rivers, the difficulties the water courses stretch for thousands of kilometers south and north of our base, our work had to be done in time free from lecturing duties, no research grants were available.

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 exploratory steps to the immediate vicinity of Khartoum. In both the White and Blue Nile we discovered the presence of a pure plankton. This was contrary to opinions expressed in the limited scientific literature available at Khartoum which stimulated our doubts and the search for the origin of this phenomenon. And so, early in our work, we became aware of the longitudinal sequence of events in running water, a fundamental feature of river ecology. of work were daunting; In the Nile, as in other long rivers, the difficulties the water courses stretch for thousands of kilometers south and north of our base, our work had to be done in time free from lecturing duties, no research grants were available.

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 exploratory steps to the immediate vicinity of Khartoum. In both the White and Blue Nile we discovered the presence of a pure plankton. This was contrary to opinions expressed in the limited scientific literature available at Khartoum which stimulated our doubts and the search for the origin of this phenomenon. And so, early in our work, we became aware of the longitudinal sequence of events in running water, a fundamental feature of river ecology. of work were daunting; In the Nile, as in other long rivers, the difficulties the water courses stretch for thousands of kilometers south and north of our base, our work had to be done in time free from lecturing duties, no research grants were available.