To understand the ecology of running waters, a concept from physics has to be adopted: the concept of field, a demarcated space in which there is an organizing factor. In running water, the field is the bed of the water course, from the source to the confluence or estuary on a longitudinal profile. The organizing factor is the current, which has a three-fold action:
— as an agent of erosion, transport, and redistribution of materials from the watershed;
— as an agent of erosion, transport, and redistribution of benthic organisms (drift);
— as an agent of transport of dissolved substances, including domestic and industrial waste, and organisms in suspension (plankton).