The ultimate project site was at first known as the Scott AFB Joint-Use Project, because this ended up being the first joint use airport in the U.S. (there had been earlier examples in Europe). Scott AFB is on the west side of Silver Creek, and the concept was to build a new runway, taxiway, and terminal east of the river. Both the military and civilian sides would share existing maintenance and control tower facilities, reducing both costs and environmental impacts. It also kept Scott AFB off the base closure list, thus building a lot of local economic support.
Even with some very creative designing to minimize impacts, the need for a connecting taxiway across the 1.5-mile wide floodplain associated with Silver Creek meant some pretty substantial impacts to second-growth floodplain forest which qualified as jurisdictional wetland. To this day it's the largest mitigation project that I've actually helped build, by about double: We mitigated 100 acres of fill at a 2:1 ratio, or 200 acres of mostly bottomland hardwood forest re-establishment.
The concept was fairly simple. The existing floodplain forest, while vast, was badly fragmented. The forest had been repeatedly cut and most of it was from 30 to 50 years old; one small section was about 90 years old and beginning to show some mature habitat structure, but mostly we were working with even age stands dominated by cottonwood, silver maple, box elder, and green ash. Agricultural fields had been cleared within the forest and leveed off. In wet years they simply flooded from the inside, and most fields produced crops only about half the time. Most of the fields were small, although the largest one, and the cornerstone of our mitigation concept, was about 70 acres. The general idea was to breach the low levees to restore connectivity to the river and the floodplain, and then to plant trees. We also jump started things by salvaging some of the larger trees cut in the taxiway area, and placing them upright in the largest mitigation site as snags. The idea was to get structure more typical of mature forests in place right away. Reducing fragmentation was a primary goal, and with the help of USACE-WES and USFWS we applied a then draft Bottomland Hardwood Forest Community HSI model to document existing and predict future conditions.
We also took an opportunity to re-meander the river and an associated tributary. The northwest end of the new runway required relocating part of Silver Creek, in a reach just south of Interstate 64 where it had been straightened and channelized. The new design included sweeping meanders similar to those intact in the southern part of the project site. We planted willow stakes to stabilize the meanders and to encourage formation of a riparian zone, and included some in-channel structure in the design.
Mitigation construction began in October of 1993, and the impact side of the project was built in 1995-1996. Most of the mitigation was completed at the same time, with a Phase 2 area north of I-64 built a few years later to compensate for some impacts that were also delayed.
We experienced heavy flooding in 1997, the same well known Mississippi River events that flooded parts of the St. Louis riverfront and destroyed entire small towns such as Valmeyer (our engineers helped design the relocation of Valmeyer to the top of the bluff). Our new meanders held, although the in channel structure was swept away and replaced with natural woody debris. Then came some very dry summer months. In spite of all this the mitigation held up fairly well. The design was flexible enough to allow for stochasm. It was resilient, a term not much in use yet at that time. That experience has influenced my restoration concepts ever since.
What would we have done differently? The agencies asked for relatively homogenous oak and hickory plantings, and we didn't argue as hard as we should have. In practice the oaks and hickories did pretty well on the somewhat drier terraces, and not well at all on the lowest and wettest areas which grew up instead with willows and other soft mast species. This turned out to be OK, because when I went to the Corps with monitoring results and aerial photos documenting excellent interspersion of habitat structure, we were released from permit conditions for the Phase 1 part of the effort. Everyone, agencies and consultants, learned from the experience. That was in 2005, which was the last time I visited the site; someone else completed the Phase 2 monitoring.
So the only thing I'd really change would be to not waste money by planting hard mast trees in the wettest areas.
We documented increased biodiversity. Fish sampling resulted in mainstem IBI scores returning to just above pre-project levels and tributary IBI scores well above pre-project and into the very good range. Amphibian species richness increased slightly (I published this data a few years later), and state-listed red-shouldered hawks nested in our snags the first year.
This photo shows the largest mitigation unit as of 2013; maybe someday I'll return to the site again, but that will require coordination with and permission from airport security. So for now I'll rely on remote sensing. That's Silver Creek on the left, and some of the more mature remaining forest is on the left of the river. The mitigation site (about the center third of the photo, from the river to that vertical green slash within the brown just right of center, that's a ditch which bounded one side of the site) continues to show good interspersion, although it looks like there's been some recent flooding. The pond just left of center was excavated as part of the project to mimic an oxbox pond. The small more densely wooded area in the top right is a pre-project stand of then very young trees, about 20 years older than the mitigation plantings. The pale green area in the very bottom center is the wettest part of this site, and is unlikely ever to support more than a few large trees.
There was one other lesson, one I don't have an answer to. Some of the suburbs nearby, especially along the interstate corridor, sprawled post-project. Where three towns meet, retail developers played the towns off against each to get concessions. The result was shopping malls with no pedestrian friendly features, and only the bare minimum of stormwater retention. They represent the worst of suburban sprawl and car culture. Those places were beyond our planning control, and yet in the long term they will affect local water quality as well as quality of life. It's a policy dilemma that's larger than any one project.
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