In their natural state, rivers are not separate or separable from surrounding lands. Rather, a river channel is just one integral part of a larger river-floodplain ecosystem. Annual flood pulses and larger flooding events connect river channels to their floodplains, driving the cycles of life for the particularly diverse ensemble of species that live in floodplain ecosystems. For example, fish use floodplain lakes and backwaters for spawning, shelter, feeding and nurseries. Plants on the floodplain depend on nutrients supplied by sediment deposited during flooding. Due to their geological age, size, habitat complexity, and variability, large river ecosystems – such as the Amazon basin – are among the more biodiverse ecosystems on Earth.
Building levees to contain river water eliminates annual flood pulses, thereby fracturing an ecosystem dependent on these processes. Therefore, for example, “in both tropical and temperate rivers, fish yield per acre is considerably greater in rivers with flood pulses and floodplains than in nearby impoundments where flood pulses are reduced or absent” [Sparks 1995: 172]. In addition,
On land, the natural nutrient-replenishment system once provided by the flood must be replaced with commercial fertilizer. Some societies practice a flood-adapted form of agriculture or harvest both fish and a compatible crop, such as rice, but intensive, high-yield agriculture often conflicts with fisheries, particularly if pesticides are used that can contaminate fish through biomagnification [Sparks 1995: 172].
To at least partially reconnect rivers with floodplains, the author recommends modifying existing structures to divert some flow to create or maintain side channels into the floodplains and restore the annual flood pulse.
Sparks, Richard E., 1995, Need for ecosystem management of large rivers and their floodplains: these phenomenally productive ecosystems produce fish and wildlife and preserve species, BioScience 45:3, https://www.jstor.org/stable/pdf/1312556.pdf?seq=1#page_scan_tab_contents.