MISSISSIPPI RIVER DIVERSIONS AND COASTAL RESTORATION Christopher M. Swarzenski United States Geological Survey, La Water Science Center, Baton Rouge

SOIL IS THE PRIMARY METRIC TO EVALUATE EFFECTIVENESS OF ANY RESTORATION APPROACH - Quality: resists tendency to erode, such as from tides and storm surge - Quantity: accretes vertically fast enough to keep pace with relative sea level rise (subsidence+eustatic sea level rise) NUTRIENT REQUIREMENTS OF COASTAL MARSHES NO peer-reviewed science that an insufficient supply of nutrients has contributed to wetland loss in coastal Louisiana or anywhere else in the world NO peer-reviewed science that adding nutrients helps or has helped reduce rates of marsh loss in coastal Louisiana or anywhere else in the world

CHENIER PLAIN MISSISSIPPI DELTA PLAIN Active Atchafalaya River Active Bird’s foot Delta Inactive New Orleans

WITH MINERAL MATERIAL, Recently deposited mud West Bay, August 2011 inorganic, heavy, inanimate WITH ORGANIC MATERIAL Eroding shoreline, Lake Salvador highly organic, light, “living” Building and Maintaining Delta Wetlands ACTIVE DEPOSITION IN SITU ACCRETION

TYPES OF RIVER DIVERSIONS I. WATER-QUALITY a. “rejuvenation” e.g. Caernarvon b. “maintenance” e.g. Davis Pond examples Caernarvon, Davis Pond, Terrebonne basin II. MINERAL SEDIMENT (land-building) examples include Wax Lake Outlet, West Bay, Bird’s Foot delta

“REJUVENATION” Purpose Transform a degraded (marsh) soil into something less degraded => higher-quality soil Constituents in River Water -fresh water (reduces salinity) -fertilizers (nitrate-N, orthophosphate) -sulfate -herbicides -but very few clays and silts

What we might hope for when we “rejuvenate” a marsh Water-quality diversion Existing degraded S. patens marsh soil Soil of S. falcata marsh, typical of new salinity regime

“The diversion isn't filling the marsh with sediments on a grand scale”. But the effect of the added river water—loaded as it is with fertilizer from farm runoff—is plain to see. "It turns wetlands hanging on by the fingernails into something quite lush" “Those nutrients are necessary for nourishing the marsh environment” SCIENTIFIC CONSENSUS “Two diversions on the Mississippi are already demonstrating how well diversions can work to restore wetlands. Although these are freshwater diversions (meaning they pull water off the top of the river rather than the bottom, where heavier sediment loads are) and thus do not provide a lot of sediment to the wetlands, they are still beneficial”

REALITY: Water quality diversions appear to compromise soils I. Swarzenski et al 2005, 2008, freshwater Panicum marshes Chronic passive inflow of Lower Atchafalaya River water since 1950s coincided with massive conversion of freshwater Panicum marsh to open water and degraded marsh, soils more decomposed than reference marshes, observation, linked to river water influx Biogeochemistry 90, p II. Kearney et al brackish S.patens marshes Caernarvon, White Ditch and Pte a la Hache Post Katrina, areas within outfall were more eroded than adjacent reference marshes at all three controlled diversions; no data on causes, remote sensing, correlation Geophysical Research Letters 38 L16405 III. Deegan et al, 2012, New England S. alterniflora salt marsh 9 years of nutrient additions: plants produced fewer roots, organic matter decomposed more rapidly and creek banks collapsed, experimental, causative Nature 490, p

I. Demise of Penchant Panicum marshes ( ) pre Rain is primary source of fresh water Flow east to west Solid expanse of P. hemitomon River is primary source of fresh water Flow west to east Loss of > 60% of P. hemitomon marsh Swarzenski et al Biogeochemistry 2008

Three to six months of passive river water influx for 40+ years (Lower Atchafalaya River example)

Soil Quality: with long-term (40+ years) river water influx decomposition appears enhanced (same plant community, Panicum hemitomon) - No difference in year-end standing biomass - No difference in accretion rates -More organic matter in marsh receiving river water BUT: - Soil is much more decomposed - links to river water include sulfate, nitrate, alkalinity, pH River water subsidyReference Swarzenski et al Biogeochemistry 2008

OBJECTION: NUTRIENTS GOOD NUTRIA BAD Nutria are the problem, herbivory is stimulated by nutrient enriched plant material, nutria multiply out of control and marsh dies off Photo from Holm et al 2011, LPBF

RiverReference/Rain SEPTEMBER ABOVE-GROUND STANDING BIOMASS (n=4) grams m 2 504(30) Total 538(31) 337(27) Panicum 441(31) SOIL (n=3) Stock (kg C,.5 m by 1 by 1 m) Accretion, (g C Per m2 per year) Swarzenski et al Biogeochemistry 2008

Outfall Hydrologic barrier Caernarvon II. Controlled diversion at Caernarvon, post Katrina Kearney et al 2011, Geophysical Letters

Outfall Marsh East Control in direct path of Katrina MARSHES POST KATRINA WITHIN OUTFALL AND ON EAST SIDE OF DELACROIX RD Kearney et al 2011, Geophysical Letters

OBJECTION: Pattern of loss is random, or energy was concentrated on west side, but not on east side of Delacroix Road PRE KATRINA POST KATRINA Kearney et al 2011, Geophysical Letters

“ By contrast, when nutrients were added, the authors observed that plants produced fewer roots, organic matter decomposed more rapidly and creek banks collapsed, leading to wider creeks and less vegetated marsh, with narrower bands of S.alterniflora” III. DEEGAN ET AL, 2012, Nature

OBJECTION: Deegan et al 2012, Nature New England salt marshes are different -Macrotidal environment (5-7 ft tides) -Ice formation (erosion)

1. WEAKENED SOILS ARE THE COMMON THREAD - Correlational (field observations, remote sensing) and experimental studies in different marsh types from Atlantic and Gulf Coasts point to similar outcomes, weakening of soils: in areas with nutrient enrichment, soils are more decomposed and erode more readily during daily tides and during storm events - excessive flooding could also contribute to weakening of soil 2. ABSENCE OF EVEN A SINGLE STUDY SUGGESTING SOIL IS GAINING SHEAR STRENGTH WITH RIVER WATER SUBSIDIES Conclusions (Water-quality diversions)

TYPES OF RIVER DIVERSIONS I. WATER-QUALITY a. “rejuvenation” b. salinity control examples Caernarvon, Davis Pond, Terrebonne basin II. MINERAL SEDIMENT (land-building) examples include Wax Lake Outlet, West Bay, Bird’s Foot delta

Scientific consensus is that massive sediment diversions off of mainstem river, in mid-Barataria Basin, are key to restoring coast What is a massive sediment diversion? 50,000-80,000 cfs controlled ? 1-2 months per year?? 3-4 months per year?? >250,000 cfs uncontrolled?? Is mid-Barataria Basin a suitable location for massive sediment diversions in terms of initiating processes for a sustainable coast?

EVIDENCE THAT SEDIMENT DIVERSIONS OFF THE MAIN STEM WILL WORK: WAX LAKE OUTLET "To say that there's not thriving vegetation there with the exact same water that's in the Mississippi River is just a fundamentally flawed statement," NAS, NWF, EDF, Wax Lake Delta: a diversion that works

accretion (cm/year) Mineral sediments, kg in top 24 cm per m Birds Foot LAR/WLO Significantly more mineral sediment is deposited in the Bird’s foot delta wetlands than in the Wax Lake/Lower Atchafalaya River wetlands

THE BIRD’S FOOT DELTA IS NOT BUILDING LAND

WAX LAKE OUTLET IS NOT A GOOD ANALOG FOR SEDIMENT DIVERSIONS OFF OF THE MAIN STEM MISSISSIPPI RIVER SYNERGY Combined 1/3 of the Mississippi River flow and about ¼ of the sediment load PLATFORM previous delta building occurred several thousand years ago, many years of marine reworking has built a firm platform to support weight of new mineral deposits SUBSIDENCE AND COMPACTION Rates are probably much lower than in Barataria and Breton; Holocene layer is much thicker in latter area

Conclusions I. WATER-QUALITY a. as “rejuvenation” problematic, appears to exacerbate marsh loss; adaptive management calls for refinement of approach, detailed studies before being implemented b. as “maintenance” works in short term, but only needed occasionally II. MINERAL SEDIMENT (land-building) - specifics are needed: size, location, key uncertainty is a better understanding of weight bearing capacity of underlying strata

SHOULD DIVERSIONS PLAY A ROLE IN COASTAL RESTORATION Emphatic yes, QUESTION IS HOW? Clarity is needed on what the expectations are for main-stem river diversions as proposed in Master Plan and then a transparent scientific debate whether these expectations are reasonable given our geological and ecological knowledge.