Editing Levee (section)

==Negative impacts==
Large scale structures designed to modify natural processes inevitably have some drawbacks or negative impacts.

; Ecological impact
: Levees interrupt floodplain ecosystems that developed under conditions of seasonal flooding.<ref>Knox, R.L., Wohl, E.E., and Morrison, R.R., 2022, Levees don’t protect, they disconnect: A </ref> In many cases, the impact is two-fold, as reduced recurrence of flooding also facilitates [[land-use change]] from forested floodplain to farms.

; Increased height
: In a natural watershed, floodwaters spread over a landscape and slowly return to the river. Downstream, the delivery of water from the area of flooding is spread out in time. If levees keep the floodwaters inside a narrow channel, the water is delivered downstream over a shorter time period. The same volume of water over a shorter time interval means higher river stage (height). As more levees are built upstream, the [[Return period|recurrence interval]] for high-water events in the river increases, often requiring increases in levee height.<ref>Munoz, S.E., Giosan, L., Therrell, M.D., Remo, J.W.F., Shen, Z., Sullivan, R.M., Wiman, C., O’Donnell, M., and Donnelly, J.P., 2018, Climatic control of Mississippi River flood hazard amplified by river engineering: Nature, v. 556, p. 95–98, doi:10.1038/nature26145.</ref>

; Levee breaches produce high-energy flooding
: During natural flooding, water spilling over banks rises slowly. When a levee fails, a wall of water held back by the levee suddenly pours out over the landscape, much like a dam break. Impacted areas far from a breach may experience flooding similar to a natural event, while damage near a breach can be catastrophic, including carving out deep holes and channels in the nearby landscape.<ref>Olson, K., Matthews, J., Morton, L.W., and Sloan, J., 2015, Impact of levee breaches, flooding, and land scouring on soil productivity: Journal of Soil and Water Conservation, v. 70, p. 5A-11A, doi:10.2489/jswc.70.1.5A.</ref> 

;  Prolonged flooding after levee failure
: Under natural conditions, floodwaters return quickly to the river channel as water-levels drop. During a levee breach, water pours out into the floodplain and moves down-slope where it is blocked from return to the river. Flooding is prolonged over such areas, waiting for floodwater to slowly infiltrate and evaporate. 

; Subsidence and seawater intrusion
: Natural flooding adds a layer of sediment to the floodplain. The added weight of such layers over many centuries makes the crust sink deeper into the [[Mantle (geology)|mantle]], much like a floating block of wood is pushed deeper into the water if another board is added on top. The momentum of downward movement does not immediately stop when new sediment layers stop being added, resulting in [[subsidence]] (sinking of land surface).<ref>Simms, A.R., Anderson, J.B., DeWitt, R., Lambeck, K., and Purcell, A., 2013, Quantifying rates of coastal subsidence since the last interglacial and the role of sediment loading: Global and Planetary Change, v. 111, p. 296–308, doi:10.1016/j.gloplacha.2013.10.002.</ref> In coastal areas, this results in land dipping below sea level, the ocean migrating inland, and [[Saltwater intrusion|salt-water intruding]] into freshwater aquifers.<ref>Schmidt, C.W., 2015, Delta Subsidence: An Imminent Threat to Coastal Populations: Environmental Health Perspectives, v. 123, doi:10.1289/ehp.123-A204.</ref>

; Coastal sediment loss
: Where a large river spills out into the ocean, the velocity of the water suddenly slows and its ability to transport sand and silt decreases. Sediments begin to settle out, eventually forming a delta and extending to the coastline seaward. During subsequent flood events, water spilling out of the channel will find a shorter route to the ocean and begin building a new delta. Wave action and ocean currents redistribute some of the sediment to build beaches along the coast. When levees are constructed all the way to the ocean, sediments from flooding events are cut off, the river never migrates, and elevated river velocity delivers sediment to deep water where wave action and ocean currents cannot redistribute. Instead of a natural wedge shaped delta forming, a "[[Bird's-foot delta|birds-foot delta]]" extends far out into the ocean. The results for surrounding land include beach depletion, subsidence, salt-water intrusion, and land loss.<ref>Edmonds, D.A., Toby, S.C., Siverd, C.G., Twilley, R., Bentley, S.J., Hagen, S., and Xu, K., 2023, Land loss due to human-altered sediment budget in the Mississippi River Delta: Nature Sustainability, v. 6, p. 644–651, doi:10.1038/s41893-023-01081-0.</ref>