Water from St. Lucie sometimes backflows into Lake O

Although the U.S. Army Corps of Engineers planned to release water from the lake to the St. Lucie starting Saturday, March 23, at an average of 250 cubic feet per second, that didn’t happen. In fact, at times water flowed the other way — from the St. Lucie Canal into Lake Okeechobee.

When the lake level is near 12 feet (above sea level), if local basin runoff brings the level in the St. Lucie Canal higher than the water level in the lake, water backflows from the St. Lucie Canal into the lake when the water control structure gates are open at the Port Mayaca Lock.

“At this point, the water level in the canal is the same as the water level in the lake. As a result, the spillway gates are open, yet water that would normally flow straight into the canal is passing back and forth between the gates,” explained John Campbell of the U.S. Army Corps of Engineers.

On Saturday, backflow at Port Mayaca was 79 cfs, into the lake, per the data report. That same day, flow at the St. Lucie lock was 206 cfs. That indicates all of the flow through the St. Lucie lock was from local basin runoff on Saturday.

Mr. Campbell said the flow rates on the corps website are estimates. “We also have a gauge downstream that measures actual flow; it has been showing that we are getting some water off the lake although it’s a relatively low volume,” he explained.

On Sunday, the flow into the lake at Port Mayaca was 83 cfs from the lake into the St. Lucie Canal. Flow at the St. Lucie Lock was 205 cfs., so 122 cfs of the flow into the estuaries was from local basin runoff. On Monday, flow at the Port Mayaca Lock was 116 cfs. The flow at the St. Lucie Lock was 242 cfs.

While the backflow situation does not happen often, it does happen regularly enough that for the past five years on average, about 2 percent of the water flowing into the lake has come from the St. Lucie Canal. The runoff into the St. Lucie Canal is higher in phosphorus than the lake water. The St. Lucie Lock is 23.9 miles from Port Mayaca. Based on the most recent five year average (Water Years 2014-2018), local basin runoff accounts for about 69 percent of the water flow and about 77 percent of the phosphorus entering the St. Lucie estuary.

The C-44 basin runoff into the St. Lucie Canal is higher in phosphorus than is the water in the lake. The runoff into St. Lucie Canal is about 347 parts per billion of phosphorus, based on the most recent five-year average. The lake water varies by location but averages around 150 ppb phosphorus.

About 95 percent of the water and about 92 percent of the phosphorus entering the lake comes from the north, according to the SFWMD data.

Publisher/Editor Katrina Elsken can be reached at kelsken@newszap.com

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