Moses Lake has received dilution water in 9 of 10 years since treatment began in 1977. The average input for those years was 143 ± 79 × 106m3/yr over 93 days, which represents flushing rates of about 13 percent/day for Parker Horn where the dilution water enters, and 1 percent/day for the whole lake. As a result, lake quality has improved at least 50 percent in terms of chlorophyllaand total phosphorus and by nearly 100 percent in transparency. Pumping diluted lake water from Parker Horn beginning in 1982 and diverting sewage effluent in 1984 markedly improved water quality in a section that received treated effluent and little or no dilution water. Total phosphorus has declined by 90 percent and chlorophyllaby 60 percent, although transparency increased only about 0.2 m because of that section's shallowness. Diluted nitrate concentration in the inflow has been considered the cause for control of algae, but a switch from largely flood to spray irrigation, as well as sewage diversion, has resulted in lower inflow phosphorus concentrations and a gradual change from N to P limitation. Despite favorable improvements in average conditions, however, blooms of blue-green algae still develop in late summer. The largest bloom in the 10 + year study occurred from late August through September 1985, when the lake was well diluted and received no sewage. This is attributed to increased mixing following lower than normal temperature and to a 50 percent increase in internal phosphorus loading. Buoyant blue-green algae rising from the sediment surface are hypothesized to be the transport mechanism.