Maurepas Swamp: What a Mississippi River Reintroduction Should Achieve

U.S. Geological Survey Fact Sheet 2018–3072, October 2018, prepared with Louisiana's Coastal Protection and Restoration Authority. By Ken W. Krauss and Stephen B. Hartley (USGS), Gary P. Shaffer, Richard F. Keim, Jim L. Chambers and William B. Wood.

In southeastern Louisiana, Mississippi River reintroductions — freshwater diversions into wetlands cut off from the river — are used to restore degraded, sinking wetlands. Until now they have all targeted marsh. Louisiana's Coastal Protection and Restoration Authority (CPRA) is now designing a 57 cubic metre per second (2,000 cubic foot per second) reintroduction to restore about 16,583 hectares (40,977 acres) of degraded, sinking swamp forest in the Maurepas Swamp. Its goal is to reduce the loss of swamp forest; reconnecting the river has often been called the most critical step to saving the swamp's natural and cultural resources.

Forest condition in the project area: closed forest canopy (undegraded), 4,991 hectares, 30 percent; transitional forest (intermediate), 9,672 hectares, 58 percent; open canopy wetland (degraded forest and open marsh), 1,920 hectares, 12 percent. Another 1,691 hectares — open water, canals and unclear habitat — were not classified. USGS, modified from Keim and others (2010) and Krauss and others (2017).

Measuring success

The swamp's decline is typical of many coastal Louisiana swamp forests. Because this is the state's first river reintroduction into a swamp forest, expectations are based on what scientists know about healthy swamp forests. The USGS, the CPRA and two universities set five performance measures (PMs) — a few specific, measurable responses to track after restoration. If a measure is not being met, the flow of river water can be adjusted; the results will also guide future reintroductions into swamp forests.

PM1 — A flooding pattern that sustains the swamp. The aim is to restore something like the flooding before levees cut the swamp off from the river — above all how often and how long it floods. Every spot should connect with the river somewhere between twice a year and once a decade. Managers must also allow drawdowns that mimic the winter and spring droughts that let tree seedlings take hold every 3–13 years — which may mean opening the diversion later in spring after a dry winter.

PM2 — Less salt. The swamp's main trees, baldcypress (Taxodium distichum) and water tupelo (Nyssa aquatica), are freshwater species that tolerate salt differently. Mean annual porewater salinity should stay below 1.3 parts per thousand where baldcypress dominates and below 0.8 where water tupelo does. Storm or drought salinity pulses are hard to manage, but more fresh water through the diversion may help.

PM3 — Ground that keeps up with the sea. Delta wetlands sink as new sediment settles and dewaters, older soils compact and the ground subsides. Cut off from the river, the swamp gets too little sediment to keep up. Its surface should rise 8–9 millimetres a year or more to keep reasonably well up with relative sea-level rise, even though the river will deliver only modest sediment; more may need to be added later.

PM4 — A healthier forest. Forest structure — tree diameters, heights and density, and the understory of plants, seedlings and saplings — is well defined for healthy swamps. Three metrics apply: stand density index should rise to 30–45 percent of the maximum for the stand type, while overstory leaf area index holds at 2.0–2.9 square metres of leaves per square metre of ground or more; and the Forested Floristic Quality Index, which rates habitat by its native species, should rise by about 12 percent — faster in the understory than among the big trees.

PM5 — Nutrients taken up, not passed on. Long cut off, with stagnant water, the swamp is low in nutrients, and Lake Maurepas carries far less dissolved inorganic nitrogen than the river. Swamp forests can use more nitrogen, but too much can cause soil loss or eutrophication downstream. Nitrate loading from river water should be about 11.25 grams of nitrogen per square metre per year — much more than now, and models suggest vegetation will take up a large share quickly.

Targets and acceptable ranges

Because swamps vary so much, each target has an acceptable range, based on the natural variation (standard deviations) in healthy swamp forests.

"OK" marks the side of the range where a result beyond it is still acceptable.

All the measures will be monitored with the CPRA, closely enough to steer how the diversion is operated.

Sources

• Krauss, K.W., Shaffer, G.P., Keim, R.F., Chambers, J.L., Wood, W.B., and Hartley, S.B., 2018, Expectations of Maurepas Swamp response to a river reintroduction, Louisiana: U.S. Geological Survey Fact Sheet 2018–3072. https://doi.org/10.3133/fs20183072 · https://pubs.usgs.gov/publication/fs20183072
• Based on Krauss and others, 2017, Performance measures for a Mississippi River reintroduction into the forested wetlands of Maurepas Swamp: U.S. Geological Survey Scientific Investigations Report 2017–5036. https://doi.org/10.3133/sir20175036
• The map, the table of targets and the full text come from the fact sheet's PDF; the web version's text is out of order. Its uncredited photographs are not reproduced.
• Words on this page from people and organisations outside the federal government are paraphrased; rewritten in hubnx's own words.