(Abstract from related journal paper prepared in cooperation with Cornell University, BEE Soil and Water Lab) Lakes hold most of the world’s freshwater resources. Safeguarding these resources from water quality degradation requires knowledge of the relationship of lake morphometry and water quality. The 3000-km2 Lake Tana in Ethiopia is one of the water resources where the water quality is decreasing, and water hyacinths have invaded. The objective of this study is to understand the lake morphometry and water quality and specifically the phosphorus dynamics and its effect on the water hyacinths. A bathymetric survey was conducted in late 2017. Various morphometric parameters were derived and both these parameters and sediment available phosphorus were regressed with the dissolved phosphorus. The results show that with a wave base depth nearly equal to maximum depth of 14.8 m, the bottom sediments were continuously suspended in the water column. As a result of the mixing, the dissolved phosphorus in the water column decreased with lake depth and increased with sediment available phosphorus (R2 = 0.84) in the northern half and only 0.45 in the south due to the large flows of Gilgel Abay. Water hyacinths were found where the lake was shallow and the available phosphorus was elevated. The large reservoir of sediment phosphorus will slow down the water hyacinth growth after the lake phosphorus input is reduced.