MARTIN COUNTY – The U.S. Army Corps of Engineers (USACE) started releasing some water through the St. Lucie Lock (S-80) on Aug. 26 as Florida nears the height of the hurricane season.

This water is NOT from Lake Okeechobee. The releases are needed to lower the level of the C-44 reservoir.

The C-44 canal, also known as the St. Lucie Canal, is part of the Lake Okeechobee Waterway, designed to allow boats to cross Florida coast-to-coast. The canal also provides flood control for part of Martin County. The C-44 canal is 23.9 miles long -- from Port Mayaca (on Lake Okeechobee) to the St. Lucie River (at the St. Lucie Lock aka S-80).

The U.S. Army Corps of Engineers tries to keep the canal between 14 and 14.5 feet above sea level, for navigation, flood control and (in the dry season) water supply. Higher canal levels would flood the adjacent areas.

When Lake O is below 14 feet, if there is a lot of rain in the C-44 basin, water managers allow water from the C-44 canal to backflow into the big lake at Port Mayaca. Data from the South Florida Water Management District (SFWMD) shows the water from the C-44 canal is higher in phosphorus than the lake water so this contributes to the lake’s phosphorus overload. However, the volume is insignificant compared to the lake.

If the lake is 14 feet or higher, water can’t be released at Port Mayaca. In years past, when this happened, the only option was to release the excess water – all local basin runoff – through the St. Lucie  lock into the St. Lucie River.

The C-44 reservoir was built to reduce that excess flow to the St. Lucie River. Since the reservoir was completed in 2022, in the rainy season when the C-44 canal levels rise, excess water from the canal is pumped into the reservoir, sparing the St. Lucie River from the excess freshwater flow.

Too much freshwater can disrupt the salinity levels in the St. Lucie estuary, which is harmful to that ecosystem.

The  3,400-acre reservoir has a pump station with a capacity to pump 1,100 cubic feet per second (cfs) of water from the canal into the reservoir.  Water from the reservoir is released through 6,300 acres of stormwater treatment area (STA). STAs are shallow marshes which allow natural vegetation to clean nutrients from the water.

The C-44 reservoir was originally designed to hold water up to 15 feet deep, however initial testing of the above ground reservoir found a potential problem with seepage at levels above 10 feet. While USACE works on ways to alleviate the seepage concerns, they are keeping it no higher than 10 feet deep.

Throughout the year, as needed, water from the C-44 reservoir is released to the C-44 STA to keep the STA hydrated.

In the dry season, water is released through the STA, which cleans the water before it flows back into the C-44 canal. Before the reservoir was built, during the dry season more water from Lake O was used to keep the canal at optimal depth for navigation and water supply.

This wet season, USACE has been using the C-44 reservoir as designed, pumping excess basin water into the reservoir. Occasionally, when rainfall is exceptionally heavy, they cannot pump it fast enough to keep the canal below 14.5 feet and do have to release some water through the St. Lucie Lock, but over all the reservoir plan has worked as designed.

Last week, as the reservoir depth neared 10 feet, USACE had to make a decision. Historically, September has been one of the worst months for hurricanes in Florida. In order to have some capacity to store more excess water should a big storm hit, they needed to lower the water level in the reservoir.

If a storm hits when the reservoir is already full, USACE would have no choice but to open the St. Lucie lock and let high volume flows from local basin runoff into the river.

On Aug. 26, the USACE and SFWMD increased releases from the reservoir to the STA to 300 cubic feet per second (cfs). Water sheetflows through the STA where natural vegetation absorbs nutrients from the water. This water passes over an uncontrolled weir back into the C-44 canal.

As needed to keep the canal level below target, water has been released through the St. Lucie Lock.

The St. Lucie River also receives flow from the C-23 basin, the C-24 basin, Ten Mile Creek and tidal basin. USACE and SFWMD take all flows into consideration.  Flow rates at any given moment can vary depending on conditions.

Rainfall is also a factor in the releases, as local basin runoff continues to drain into the C-44 canal. With Lake O too high for the C-44 water to backflow, and the C-44 reservoir already full, the only option is to release water through the St. Lucie Lock.

On Aug. 28, flow through the St. Lucie Lock was under 300 cfs. On Aug. 29, flow averaged 666 cfs.

 “While we can’t specify a timeframe for how long the releases will continue, the goal is to reduce the reservoir to approximately 7 feet in depth while maintaining S-80 releases so as to not exceed 1,400 cfs of total inflows into the St. Lucie Estuary,” explained USACE Public Affairs Officer John Paul Rebello. The 1,400 cfs target includes  flows to the estuary from all sources.