On April 25, 2013, the Merwin Preserve levee (99% at 446 ft asl) was overtopped at 447.52 ft asl which matched the historic river stage crest (447.52 ft asl) for the La Grange reach of the Illinois River in 1995. Once overtopped, the Merwin Preserve levee failed and the property filled within hours. The river stage continued to rise and crested at 448.04 ft asl on April 27, 2013, which was 0.52 ft higher than the former 1995 record. The only other time that the Merwin Preserve has been connected to the Illinois River was during a minor flooding event that overtopped the Merwin Preserve levee in 2002. The levee at the Emiquon Preserve, a larger TNC restored floodplain wetland upstream of the Merwin Preserve, has never had floodwater overtop its levee until 2013 when it experienced only a brief influx of river water because its levee is much higher (low spot at 451.3 ft asl and majority at 455.0 ft asl) and remained functional when breeched. The Merwin Preserve, in contrast, underwent a longer and more thorough inundation for 29 days. In addition, the legal requirement to move the water back out of the Merwin Preserve after the river returned to normal stage meant that another breach in the levee closer to the river had to be intentionally created. The levee failure and subsequent planned breeching allowed for the exchange of nutrients and organisms between the river and the Merwin Preserve. Because the goal of these projects is “to restore floodplain native plant and animal communities and to reconnect them to the Illinois River to allow movements of aquatic organisms”, it is important to consider how extreme, but infrequent flood events could potentially impact the dynamics of these and other floodplain restoration projects. Thus, the fish and aquatic vegetation monitoring conducted annually at the Emiquon Preserve was extended to the Merwin Preserve, following the historic spring flood in 2013to better understand how fish and aquatic vegetation respond to natural flood events and river connection within recently restored floodplain projects.
|Name||INHS Technical Report 2015 (15)|