Sanibel is a 12-mile-long barrier island located off the coast of Lee County, near the mouth of the Caloosahatchee River. Sanibel contains two large freshwater basins – the Sanibel River West Basin and the Sanibel River East Basin – which serve as essential freshwater reservoirs for the island. Protecting these freshwater systems is crucial for preserving the island’s native flora and fauna, as well as conserving the freshwater in the surficial aquifer beneath the island.

When Hurricane Ian made landfall in late September 2022, the island was inundated with storm surge, causing saltwater contamination of freshwater ponds and wetlands. Since Ian, there has been a noticeable loss of vegetation, and a general sense amongst residents that the island’s hydrology has changed.

One method to measure changes in vegetation health is through the Normalized Difference Vegetation Index (NDVI), which uses satellite imagery to assess vegetation density and greenness by comparing red and near-infrared light reflections. Johnson Engineering processed hundreds of images from NASA/USGS Landsat 8 satellite from 2013 to 2023, creating long-term NDVI averages before and after Hurricane Ian (see Figures 1 and 2). These images clearly show the hurricane’s devastating impact on plant life across the island.

Historically, water from the island’s interior basins has left through evapotranspiration, a process that is closely tied to plant growth. With a significant decline in plant populations post-hurricane, it is logical to assume that the island’s hydrology has been affected as well. However, continued monitoring shows an encouraging trend: NDVI values have steadily increased through early 2024, signaling the resilience of Sanibel’s ecosystems.

Though the island’s freshwater basins experienced further saltwater intrusion following last fall’s hurricanes, it is expected that the upcoming summer rains will replenish these basins, aiding in the continued recovery of the island’s environment.

For more information contact Jordan Varble, PE, at [email protected].