The Bonnet Carré Spillway (BCS), located about 28 miles northwest of New Orleans, was constructed by the US Army Corp of Engineers in the early 1930s as part of an integrated flood-control system for the lower Mississippi River (MR). The BCS control structure consists of 350 individual bays that can be opened to divert water from the river to Lake Pontchartrain to relieve pressure on downstream levees. Lake Pontchartrain (LP) is hydrologically connected to the Mississippi Sound (MS Sound) and the Gulf of Mexico and is more accurately characterized as an estuarine embayment. BCS openings have occurred twelve times prior to 2019 because of high Mississippi River stages, typically in late spring. In 2019, the spillway opened twice in one year, first opened from February 2019 and closing April 2019, and then reopened one month later because of a second high-water peak in May 2019. Monitoring the spillway openings and the effects of diverted water on water quality in LP and the MS Sound is vitally important to resource managers in Louisiana and Mississippi. These resources provide habitat for many species of fish, shellfish, crabs, seagrass, and other marine mammals, as well as provide recreation activities and commercial fishing. The introduction of nutrient-rich, fresh river water into the nutrient-poor, brackish LP is known to substantially change the chemistry and ecology of the lake (Mize and Demcheck, 2009). The historical 2019 BCS openings, two openings in one year, have never been recorded since the BCS was constructed. Unprecedented, large volumes of MR river water were diverted into LP and the MS Sound where lake and sound waters changed abruptly from a brackish-estuarine system to a freshwater dominated system with some areas maintaining low salinity for about 2 to 3 months. In the past, these spring openings have been associated with algal blooms later in the spring and summer. Although not normally an acute health hazard, these blooms can substantially reduce the access of the lake and sound waters for commercial and recreational uses. However, the timing and magnitude of the 2019 diversions provided nutrient flux and freshwater inflows that resulted in algal accumulations that included harmful algal bloom species with possible algal toxins in coastal zones in Mississippi. Especially hard hit was the Mississippi shoreline/beach area, where bloom status for several species of algae was recorded along with elevated algal toxin levels. The Mississippi Department of Environmental Quality issued water contact advisories for 21 different beach locations from June until October 2019 (MDEQ, 2019). Mize, S.V., and Demcheck, D.K., 2009, Water quality and phytoplankton communities in Lake Pontchartrain during and after the Bonnet Carré Spillway opening, April to October 2008, in Louisiana, USA: Geo-Marine Letters, 29, pp.431-440. Mississippi Department of Environmental Quality (MDEQ), 2019, issues water contact advisories as part of Mississippi Beach Monitoring Program <http://opcgis.deq.state.ms.us/beaches/>.