In Mississippi alone, there are 98 discharge permits along the Pearl River from Hinds County downstream to Pearl River County closer to the river’s mouth. The Pearl is the Mississippi-Louisiana boundary in its lower reaches, and Louisiana manages a number of permits on its side of the Pearl River as well. In Mississippi, permits required by the Clean Water Act, are granted by the Mississippi Department of Environmental Quality (MDEQ) to city sewage treatment plants, schools, housing subdivisions , chicken processors, slaughter houses, trailer parks, industrial parks and day care centers, to name a few. Each single discharger must treat its waste to certain standards before the effluent, mixed with water, can be returned to the Pearl or one of its tributaries. The largest permit holders such as city sewage plants may return 2-4 million gallons per day to the river, the smaller nursing homes or child care centers may send a few thousand gallons.The Pearl River is on Mississippi’s 303(d) Impaired Waters List due to excess nutrients, so a special report called a Total Maximum Daily Load (TMDL) document has been written for the Pearl River downstream of Jackson. The TMDL imposes limits on nitrogen and phosphorous in discharge permits. The Pearl’s larger discharge permits are from sewer plants , food processors or industries that produce water mixed with wastes heavy in nitrogen and phosphorous – nutrients that comes from the breakdown of organic waste whether that is human waste or comes from animal by-products. These are the same nutrient compounds that farmers and gardeners apply to fertilize their crops on land, but these compounds released in water fertilize algae that fouls rivers and streams, reduces dissolved oxygen, and kills fish. Over- fertilization causes lush growth of plants on land, but leads to heavy algae and plankton growth in water which pollutes and degrades streams and rivers.Keeping a lid on algae growth in a river or stream receiving these nutrient discharges depends on keeping the concentrations of these nutrients low, but it also depends on having enough water in the stream to mix with the waste. The MDEQ takes pains to try and impose nitrogen and phosphorous limits on dischargers when, as in the Pearl River’s case, there is an impairment and a TMDL report for nutrients. However, an important component in permits is the flow of the river into which the waste mixture is being released. Permits run into problems when flows are low or reduced and there is not adequate dilution of the released liquid waste. Flows are reduced in our rivers in the late summer and into the fall seasons each year when rainfall amounts are seasonally low. But there are other threats to flow, such as dams. Managing waste discharge permits on rivers is tough if you are also building more dams that threaten flow. The Pearl River is under such a threat now.The Pearl River was dammed when the Ross Barnett Reservoir was created in 1963. It was built as a recreational lake and to provide drinking water for the Jackson area. After the dam was built, it was discovered that large rainfall events in the upper watershed of the Pearl River could nearly overwhelm the dam. Two large floods in 1979 and 1983 tested the dam manager’s ability to operate the gates in a way that would save the dam, and limit flooding downstream of it. In doing so, backwater flooding up Jackson’s urban creeks and drainage ditches inundated parts of downtown Jackson, and many homes and businesses, causing major property damage and threats to human life.A dredging project to create a new lake on the Pearl River, the “One Lake Project” , is being considered for Jackson. Instead of modifying the levee system, the proponents of this flood control lake are saying that a wider, deeper channel over about 8 miles in the urban section of the river would transmit water more efficiently and cause less urban flooding when the Ross Barnett flood gates must be opened for large water releases. Whether levees, or a lake, or another design will do a better job in controlling flooding will soon be argued by engineers and planners, but real estate developers in Jackson are drooling over a lake and the lucrative urban waterfront they believe it will create.The economic development potential is already creating ripples. In the Mississippi Legislature’s 2017 Regular Session, ongoing right now, there is a bill (HB 1585) working its way from the state House of Representatives to the Senate that would tailor the taxing authority of the Rankin Hinds Pearl River Flood and Drainage Control District, which is sponsoring the lake idea. The tax bill would allow lakefront property to be taxed at a different rate than other property farther from the water’s edge. This tiered tax plan is clearly a way for the District to pay for the lake based on development value. And this is happening before the environmental studies for the project have even been published and offered for public comment.Whatever lakes like this one may do to property values, they reliably withhold flow and they increase evaporation. The surface area of a lake sends much more water into the atmosphere through evaporation than the surface of the river dammed to create it. These physical factors have a bearing on flow and dilution. Because they affect flow, they can reduce water volume in the river downstream of the dam. On the Pearl River, they also affect the mixing of permitted waste effluent into the river and affect the concentration of nutrients and other pollutants that will be discharged to the river at the 114 permitted discharges between Jackson and Picayune, Ms. With water being evaporated now off of the Ross Barnett Reservoir, and more evaporative loss from the surface of a second lake, the permit holders downstream should worry about what will happen to their ability to properly dilute treated discharge under their existing permits.In 2013, the engineering department of St. Tammany Parish Louisiana looked at preliminary evaporation figures and models for creating a 1500 acre “One Lake” project in Jackson and calculated that a reduction of 90 cubic feet per second (cfs) of flow would be experienced at their end of the Pearl River if the new lake project was built. That would be like removing the volume of a stream the size of Okatoma Creek.So this lake project and its effects on flow are cause for thought about what could happen to the permits relied upon by Sanderson Farms, Entergy, Georgia Pacific, and Denbury Onshore LLC, and other industries in various places where they discharge treated waste in the Pearl system ; and similarly by the cities of Picayune , Poplarville, Columbia, Foxwoth, Monticello, Mendenhall, Crystal Springs, Georgetown, Hazlehurst and Jackson, with their combined treated sewage discharges of many tens of millions of gallons per day into the Pearl River.A recent Freedom of Information Act (FOIA) file search at MDEQ revealed that Picayune’s Neal Road sewage treatment plant, probably the farthest Mississippi city downstream from Jackson and the proposed lake, is already having trouble meeting its total phosphorous limits which it exceeded in December and March of 2015, May and June of 2014, June of 2013 and in four consecutive months of 2012. When additional lake evaporation causes flow reduction problems in the Pearl River, these permit violations will be more difficult (and more expensive) to solve. A loss of 1000 acres of floodplain wetlands and forest in the dredged “One Lake” footprint, plus problems with threatened sturgeon and turtle habitat, and decreased flow effects on the swamps, estuaries and marshes of Louisiana and Mississippi should be enough to persuade decision makers that a new lake is a bad idea for the Pearl River. If not, all of the downstream businesses and cities with discharge permits should point out their nutrient limits when the lake project is published for comment later this year by the Rankin Hinds Pearl River Flood and Drainage Control District and the Pearl River Vision Foundation. With another lake on the Pearl, and the possibility of flow reductions in critical times of the year, properly diluting effluents to meet permit limits imposed by the Pearl River TMDL will be more of a challenge than ever.Andrew Whitehurst is Gulf Restoration Network’s water proram director and works on Mississippi water and wetland issues.