Sarah E Gergel

Advisor: Monica G Turner

PhD Abstract:
Human-altered disturbance regimes: effects of flood control along the Wisconsin River.

Levees have been ignored by ecologists relative to other flood regime alterations such as dams. I examine several of the generally assumed effects of levees and determine their relevance for setback levees, levees further back in the floodplain. First, a hydraulic model is used to determine whether flood-control levees have altered the pattern and frequency of inundation and flood power. Sampling of floodplain forest in the field is used to relate the simulated flood regime to changes in the floodplain forest composition. Increases in flood stage (height) due to levees were minor, only a few centimeters, primarily due to the position of the levees, set back hundreds of meters into the floodplain in some areas. Increases in overbank flood velocities due to levees wereminimal compared to increases caused by channel constriction and by increased flood magnitude. Generally, levees had a greater impact on stage and overbank flood velocities of larger magnitude events.

I also created a spatial model of floodplain inundation to examine the effects of levees and dams on teh duration and abundance of floodplain pond habitat. The relative and cumulative impacts of levees and dams on the duration of these habitats are unknown. When the levee was breached, no differences were found between the natural and levee floodplain scenarios. Dams either decreased, increased, or had no effect on the area occupied by temporary ponds, depending on flood magnitude. Synergistic interactions between levees and dams were apparent for larger flood events where the reduction in flood stage due to dams preventing levee breaching. Lastly, building on the flood model, I determine how levees and dams have impacted denitrification (or water purification) services provided by floodplain ponds and wetlands. We compared three scenarios: a natural river-floodplain, a leveed floodplain, a regulated river-floodplain. Total NO3 processing increased with flood magnitude. The percent processed under the three scenarios was similar, also suggesting that the initial mass of NO3 on the floodplain was critical in controlling the total NO3 processed.

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