Pathways of river nutrients towards the euphotic zone in a deep-reservoir of small size: Uncertainty analysis

The ecological consequences of river inflows, usually carrying large amounts of nutrients, will be different depending on whether it penetrates to the hypolimnion, inserts in the metalimnion or remains on the surface. For plunging rivers, the intrusion depth is controlled by the river-reservoir density difference prior to the plunge point together with mixing processes between ambient and inflowing water, which occur both in the region of the plunge and after the flow has assumed the form of a density current. In contrast with the processes of entrainment into density currents, which have been extensively studied, entrainment and mixing within the plunge zone has received less attention in the physical limnology literature. The existing literature fails to identify adequate parameterizations of this phenomenon, while exhibiting a large variability in the initial mixing rate, measured both in the laboratory and in the field. This work examines the consequences (in terms of uncertainty) that our lack of knowledge of parameters describing river-reservoir mixing in 1-D transport models has on the estimates of river nutrient loads into the euphotic zone of a reservoir, where nutrients are readily available for phytoplankton growth.