Releases to ‘lower lake’ have little effect on Big O level

Residents on both coasts have what they asked for in regard to flow from Lake Okeechobee, at least for now.

There has been no flow to the St. Lucie Canal from the lake since March 30. The target flow has been set at “zero” by the U.S. Army Corps of Engineers, which controls that structure.

Releases from the lake to the Caloosahatchee River are currently set at a maximum of 1,000 cfs by the corps. The 1,000 cfs is the ideal beneficial target for dry-season flow, requested by Lee County officials, to benefit the Caloosahatchee Estuary and prevent it from having salinity levels that are too high in the dry season.

The lock at Port Mayaca is open, explained John Campbell of the U.S. Army Corps of Engineers. He said the level of water in the canal is the same as the level of the lake, so while there is some sloshing back and forth, there is no flow from the lake at Port Mayaca.

The canal connecting Lake Okeechobee and the St. Lucie River is manmade. Flow at Port Mayaca is gravity-driven. If the level of Lake Okeechobee is lower than the level of the water in the St. Lucie Canal, water backflows into Lake Okeechobee. The St. Lucie Lock is 23.9 miles from the Port Mayaca Lock.

The first weekend in April, the water control structures indicated there was some backflow from the canal into the lake, at the rate of 89 cfs on April 5 and 175 cfs on April 6. Mr. Campbell said those were false readings due to some issues with one of the gauges. He said there was no measurable flow.

In February and March, the U.S. Army Corps of Engineers released water to the St. Lucie Canal and the Caloosahatchee River to attempt to lower the level of Lake Okeechobee in anticipation of the wet season. The theory is that dry season releases from the lake could prevent the need to release as much water during the wet season.

So in February and March, flow to the Caloosahatchee River, which needs regular flow in the dry season for the health of the Caloosahatchee Estuaries, was increased and some lake water was released to the St. Lucie Canal, which does not need any flow from Lake Okeechobee.

How much difference did those releases make? About 2 inches on Lake Okeechobee.

Let’s do the math …

From the start of the dry season in October until Feb. 23, no lake water flowed into the St. Lucie Canal.

From Feb. 23 to March 16, flow to St. Lucie was 500 cubic feet per second. That’s a total of 21 days for total flow of 6,786,327,261 gallons.

From March 16 to March 23, flow to St. Lucie was 250 cfs. That’s a total 1,131,054,540 gallons for the seven-day period.

From March 23 to March 30, the target was 250 cfs. That’s 1,131,054,540 gallons for the seven-day period.

That brings total flow to the St. Lucie so far since Oct. 5, 2018 to 9,048,436,341 gallons released from Lake Okeechobee to the St. Lucie.

However, all of that water was not from the lake. The water is measured at the St. Lucie Lock, which is 23.9 miles from the Port Mayaca Lock were the water enters the St. Lucie Canal from Lake Okeechobee. So it’s actually less than 9 billion gallons, but just assume the whole 9 billion gallons was from the lake.

Now look at the Caloosahatchee releases. Starting on Oct. 19, the corps has been releasing water to the west, at the request of Lee County officials. The South Florida Water Management District has guaranteed the river 400 cfs flow during the dry season, to help prevent saltwater intrusion. Lee County officials have asked for a minimum of 750 to 800 cfs., even filing a lawsuit over it, and have stated 1,000 cfs would be the ideal dry season flow to the estuaries. Too little freshwater flow makes the estuaries too saline.

In an effort to improve the health of the Caloosahatchee Estuaries, starting in October, the corps has released 800 to 1,000 cfs to the Caloosahatchee River, as measured at the Franklin Lock. The Franklin Lock is 43.4 miles from the Moore Haven Lock, where lake water enters the river. Along that 43.4 miles, local basin runoff also enters the river. So that flow at the Franklin Lock is a mixture of lake water and local basin runoff.

From Oct. 19 to Jan. 11, Caloosahatchee was 1000 cfs. That’s 54,290,544,172 gallons of flow, all to benefit the estuary at the request of Lee County officials.

From Jan. 11 to Jan. 25, Caloosahatchee flow was 850 cfs. That’s 7,691,170,900 gallons, also beneficial to the estuary.

From Jan. 25 to Feb. 1, flow was 700 cfs, for a total of 3,166,952,726 gallons.

Feb. 1 to Feb. 23 flow was 1,000 cfs for a total of 13,572,654,543 gallons.

From Feb. 23 to March 23, flow to Caloosahatchee was 1,800 cfs. Beneficial flow of 1,000 cfs totals 18,096,872,724 gallons; extra to lower the lake totals 14,477,498,168 gallons.

From March 23 to March 30, the target for the Caloosahatchee was 1,400 cfs. That’s a beneficial flow of 1,000 cfs at 4,525,218,181 gallons total and, extra flow to lower the lake of 400 cfs, totaling 1,808,687,271 gallons.

Beneficial flow to the Caloosahatchee River since Oct. 19 totals 103,152,100,517 gallons or about 8.6 inches of water on Lake Okeechobee
Extra flow from the lake to lower Lake Okeechobee level totals 16,286,195,439 or about 1.4 inches of water on Lake Okeechobee.

To summarize the totals: The water released to the Caloosahatchee River from Oct. 19 to March 30, at the request of Lee County officials for the benefit of the Caloosahatchee estuaries, totaled about 8.6 inches of water on Lake Okeechobee.

The extra water released to the Caloosahatchee River to help lower the lake level totaled about 1.4 inches of water on Lake Okeechobee.

Those numbers assume all of the water that came through the Franklin Lock and the St. Lucie Lock came from Lake Okeechobee. History shows us some of the water that comes through those locks is local basin runoff; how much depends on rainfall in those areas.

So the U.S. Army Corps of Engineers efforts to lower Lake Okeechobee by releasing water to the C-43 and C-44 canals in February and March lowered Lake Okeechobee by a total of 2.15 inches.

The extra releases have stopped (although water still flows to the Caloosahatchee for the benefit of that estuary), and yet the lake level is dropping. Why? Evaporation and percolation are responsible for most of the water that leaves the big lake every year. Water evaporates into the air and percolates through the earth into the aquifer year-round. During the wet season, the loss to evaporation and percolation are offset by water flowing in. During the dry season, with less flow into the lake, the lake level drops. The lake needs the seasonal highs and lows. The lows allow the grasses in the marshes around the edges of the lake to grow, and when the rains come, those areas are flooded, providing spawning areas for fish and cover for small fish. The marshes are also the lake’s filter system, removing nutrients like phosphorus and nitrogen from the lake.

Water from the lake also flows south, which is its natural flow path, although due to flood control, it now flows through canals instead of sheetflowing over the edge of the lake. Water that flows south from the lake irrigates farms, provides flow to the Water Conservation Areas that send clean water to the Everglades and provides aquifer recharge and some water supply for cities on the lower East Coast.

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