CIVE 633 - ENVIRONMENTAL HYDROLOGY

THE FLOOD PULSE CONCEPT

  • Hydrologists think of rivers as longitudinal links in the hydrological cycle.

  • However, there are significant transversal components in large rivers.

  • River-flood plain systems provide important habitat for biota.

  • Ecology is traditionally divided into terrestrial and aquatic.

  • Water bodies are traditionally divided into lentic (lakes, ponds) and lotic (streams, rivers)

  • Flood plains do not classify readily into either of these classifications.

  • ATTZ: Aquatic-terrestrial transition zone is used to describe the seasonally inundated flood plain.

  • Evidence in large rivers supports a concept different from the RCC (longitudinal): the flood pulse concept (transversal).
THE FLOOD PULSE CONCEPT

  • The flood pulse is the major force controlling biota in river-floodplain ecosystems.

  • Lateral exchange is more important than the longitudinal (upstream) supply postulated by the RCC.

  • In large river with flood plains, the majority of the animal biomass derives from production in the flood plain and not from the downstream transport.

  • Whether a river has a flood plain depends on stream order, geology, geomorphology, discharge, and continental location.

  • Most tidal rivers have flood plains.

  • Rivers with geologically induced continental deltas, like the Upper Paraguay, have extensive flood plains.

  • Runoff diffusion will determine to a large extext the existence of a flood pulse and consequently, a healthy flood plain.

  • Runoff diffusion is a function of the prevailing slope; the smaller the slope, the more the runoff diffusion, and the more the likelihood of the existence of the flood pulse, regardless of continental location (Examples: the Pantanal, the Ohio).
DEFINITION OF A FLOOD PLAIN

  • The "active floodplain" of a river is defined in hydrology as the area flooded by a 100-yr flood.

  • This period is arbitrary, and has little ecological meaning.

  • Junk defines floodplains as "areas that are periodically inundated by the lateral overflow of rivers and lakes."

  • The resulting environment causes the biota to respond to morphological, anatomical, physiological, phenological (relation between climate and periodic biological phenomena, as in bird migrations or flowering), and ethological (study of animal behavior) adaptations, and produce characteristic community structures.

  • The floodplain area is the ATTZ because it alternates between aquatic and terrestrial environments.

  • The hydrologic unit is termed the "river-floodplain system."

  • The river-floodplain system includes lentic, lotic and ATTZ.

  • References to origin of energy (allochthonous and autochthonous) should be qualified in the ATTZ.

HYDROLOGY

  • The number of floods per year will depend on the size of the basin, climate, precipitation patterns, geomorphology, and continental location.

  • Systems with large runoff diffusion will have a tendency to develop one or two flood peaks per year (Amazon, Paraguay)

  • The flood pulse is a characteristic of large rivers with seasonal floods interacting with the floodplain.

  • The flood pulse causes change to the ecological norm in the ATTZ.

  • This change enhances productivity, species diversity, and energy flows.

  • Interruption of the flood pulse by anthropogenic action (levees) will change the character of the ATTZ to terrestrial ecosystem, reducing productivity, species diversity, and energy flows.

CONCLUSIONS

  • Flood plains represent transition zones (ATTZ) that alternate between aquatic and terrestrial states.

  • A pulse of long duration is characteristic of unmodified large rivers.

  • The flood pulse maintains the ATTZ in dynamic equilibrium.

  • Aquatic and terrestrial productivity of river-floodplain systems depends on the nutrient status of the water, the climate, and the flood pulse.

  • The FPC is dominant over the RCC in systems with floodplains (ATTZs), particularly when the pulse is regular and of long duration.

  • Processes in floodplains do not depend on inefficient processing of organic matter upstream.

  • The FPC should help in the hydroecological understanding of river-floodplain systems.

 
041027