Lakes hold most of the freshwater resources in the world. Safeguarding these valuable resources in a changing environment is a major challenge. The 3046 km2 Lake Tana in the headwaters of the Blue Nile in Ethiopia is one of these lakes. The shallow (9 m deep) lake which was pristine in the 1930s has become increasingly polluted with sediment and nutrients from the intensifying agriculture using increased amounts of fertilizers. Water hyacinths began to appear ten years ago mainly due to increased phosphorus concentration. Little is known about the practices to improve lake water quality. The objective was to investigate sediment and phosphorus dynamics and with the ultimate goal to better understand steps that could be taken to improve water quality and to prevent the spread of water hyacinths. A lake bathymetric survey was carried out in 2017. The survey was compared with earlier surveys to obtain sediment deposition rates; In addition, water and bottom sediment samples were collected; Sediment samples were analyzed for texture and available phosphorus, total nitrogen, and organic matter and the water samples, the concentrations of dissolved phosphorus and suspended solids were determined. Water flow direction in the lake that was simulated with the Delft3D model showed that water, dissolved nutrients, and part of the sediment of the Gilgel Abay (the largest river entering the lake at the south) flowed to the Tana Beles Tunnel and when the lake was at its maximum operating level to the Blue Nile River. The water of Rib and Gumara rivers entering from the east and northeast circulated counter-clockwise and the flow of the Megech river from the north circulates clockwise. Consequently, sediment and phosphorus from the three rivers were transported to the northeast. Most of this water was lost by evaporation and the sediment and phosphorus settled to the lakebed mainly in the northeastern part of the Lake. Comparing the four bathymetric surveys conducted in 2017, 2006, 1987, and 1940, Lake Tana’s depth decreased with an average of 28±11 cm equivalent to 900±0.4Tg (million tons) of sediment deposition since the 1940s. Most of the sediments were deposited in areas near the major river inlets. As a result, two to ten km long peninsulas were formed. The deposited sediments were mostly clay. In the shallowest areas, the sediment texture was courser while in the deeper part, center, northwest, and the west were fine clay. The bottom sediments had a mean organic matter content of 16 g kg-1, total nitrogen 0.8 g kg-1, and Olsen available phosphorus was 19 mg kg-1. The morphometric parameters derived from the bathymetric survey show that the wave base depth was equal to a maximum lake depth (14.8 m). This means that the bottom sediments were continuously resuspended in the water column. As a result of the resuspension, the dissolved phosphorus in the water column decreased with the lake depth and increased with sediment available phosphorus (R2 = 0.84) in the northern half of the lake. The relationship was not as strong in the southern part due to a large flow of Gilgel Abay to the two outlets. Consequently, the lake that was shallowest in the northeast had the highest dissolved phosphorus concentrations The research found that the lake morphometry and circulation pattern could explain the location of the water hyacinths found in the northeastern part of the Lake. The lake circulation is such that the phosphorus-rich sediments of three of the four main rivers are transported to the northeast. As a result, the northeastern part of the lake is shallow and bottom sediments are high in phosphorus, explaining the elevated dissolved phosphorus concentration in the lake water and the location of the water. In addition, water hyacinth growing in other parts of the lake float to the northeast partly due to lake circulation and partly due to the prevailing southwestern winds.