CIVE 633 - ENVIRONMENTAL HYDROLOGY
ECOSYSTEM MANAGEMENT OF LARGE RIVERS AND THEIR FLOODPLAINS
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- River floodplain ecosystems are characterized by seasonal floods that promote the exchange of nutrients and organisms
among a mosaic of habitats and thus enhance biological productivity.
- Annual flood pulses are so predictable that animals and even humans have adapted to take advantage of them.
- Replenishment of floodplains with fresh nutrients has permitted the development of great civilizations.
- In the U.S., most of the 98,000 sq km floodplain along the Mississippi downstream of the Ohio has been leveed and drained for agriculture.
- Two large tributaries, the Illinois river, and the lower portion of the Missouri retain flood pulses and floodplains.
- In 1993, disastrous floods focused attention on the ecosystem management of floodplains.
- The concept that large river floodplain integrity is maintained by hydrological dynamics (flood pulse) is a relatively recent idea.
- Lotic ecology has focused on smaller streams.
- Environmental agencies have focused on water quality in the channel.
- The U.S. Clean Water Act established a goal of "restoration of the physical, chemical, and biological integrity of
the nation's waters..."
- EPA subsequently concentrated on the chemical part of this integrity triad.
- Ecosystem management works with the natural driving forces and variability with the goal of maintaining and recovering biological
IMPORTANCE OF LARGE RIVER-FLOODPLAIN ECOSYSTEMS
- Large river-floodplain ecosystems are often rich, with reasons having to do with their age, size, habitat complexity, and variability.
- In the tropics, the flood pulse provides a seasonal change in an environment which is otherwise unseasonal.
- Species richness not only depends on speciation but also on avoidance of extinction.
- North-south orientation of the Mississippi provided an escape route to southern fresh water refuges.
- The Mississippi flyway is the migration corridor for 36% of North America's waterfowl.
- Fish undertake longitudinal (along the channel) and lateral (across the floodplain) migrations to spawning and feeding areas.
- Optimal conditions vary within the flood cycle.
- Some quiescent backwater basins are thermally stratified, with denser warmer water on the bottom.
- Large rivers transport water, sediment, nutrients and contaminants.
- Rivers can have large-scale eddies, bedforms, and woody debris that retard the movement of organic matter, sediments and nutrients.
- Floodplains provide great opportunity for the retention of nutrients.
- 35% of the total nitrogen entering the Mississippi is retained or lost through denitrification before reaching the sea.
- In both tropical and temperate rivers, fish yield per acre is considerably greater in rivers with flood pulses than in rivers
with no pulse.
- If the water recedes too rapidly off the floodplain, organic matter, nutrients and newly hatched aquatic organisms may be carried into the
CONSEQUENCES OF ALTERING NATURAL PROCESSES
- Leveeing floodplains to prevent flooding eliminates the flood pulse advantage in the ATTZ.
- Rivers that are channelized or leveed lack the vegetated riparian buffers and floodplains that could take up excess nutrients loads.
- Nitrogen load at the mouth of the Mississippi now exceeds the presettement load by two to three times, due to increased fertilizar loadings.
- These nutrients contribute to plankton blooms in the Gulf of Mexico.
- Downstream sediment transport can be interrupted by sediment trapping behind dams.
- The issue of whether levees cause aggradation or degradation has not been resolved.
- Aggradation is caused by long periods of low flow.
- Degradation is caused by floods.
- Levees with upstream dams lead to long-term aggradation, requiring higher levees.
- Flushing may be necessary to restore the system to equilibrium.
- Levee fallacy: levees will increase flood stage and cause scour ("self cleaning" argument).
- The Stevens statement: Levees increase flood stages!
- Ecosystem management recognizes the functional relationship between living resources and physical features of the landscape.
- Ecosystems management is more than the traditional "species focused" approach.
- The species-focused approach focuses on habitat rehabilitation and enhancement.
- There is not enough information on the flood-plain ecosystem to develop valid habitat-suitability models.
- Optimizing habitat for a few highly valued species may create suboptimal conditions for other species.
- Levees may block currents and interfere with fish migration to spawning areas.
- If high levees are maintained, the flood plain cannot fulfill its hydrological function of conveying and storing major floods.
- Flood heights and damages increase everywhere.
- The ongoing restoration of the Kissimmee river in Florida is notable because competing interests agreed on the goal of
restoring the natural hydrological regime and river configuration that has once (1970) all the native species.
- They specifically rejected an impounded, highly managed system.
- The Kissimmee restoration exemplifies the ecosystem management approach.
- The goal of ecosystem management is to maintain or recover the biological integrity of the ecosystem.
- Biological integrity is the capability of supporting a balanced community of organisms of characteristics similar to that of natural
- In large alluvial river-floodplain ecosystems, the primary abiotic factor affecting biotic integrity are water and sediment quality and the
temporal patterns of water and sediment flows that shape the river channel and the floodplain.
- These factors strongly influence habitat structure, trophic base, and biotic interactions.
- Master processes include the abiotic processes of erosion and sedimentation that maintain floodplains and deltas.
- Key questions are how to determine the natural patterns and how to preserve them or restore them within the constraint of human needs.
- Altered flood pulses can initiate long-term changes in the ecosystem, changes which are difficult to reverse.
- The combined predictive expertise of fluvial geomorphologists and ecologists is needed for ecosystem management.
- Raising the navigation dams to deepen the lakes is merely likely to increase the sediment-trapping efficiency and the wind fetch,
so they will fill with sediment faster.
- Ecosystem management works with natural processes such as erosion, sedimentation, and seasonal flood pulses, attempting
to guide them rather than thwart them.
- Ecosystem management can restore natural services such as flood storage, water purification, fish production, and biodiversity.