Camp Dodge Wetland Study
by C.A. Thompson
This project began in 1995 and is a hydrological study of an existing and restored wetland on the Camp Dodge National Guard Base (Figure 1). Researchers from several universities and state and federal agencies are studying the site's hydrology, water quality, flora, and fauna. An existing wetland at the site was instrumented with hydrologic equipment in order to serve as a reference site for the restoration. The restoration site had been artificially drained by a tile system which was cut at several locations in June 1996.
The wetlands are partially closed systems; the primary input
is precipitation and the primary output is through
evapotranspiration. However, it is important to recognize that
these wetlands are also connected to larger flow systems.
Recognition of these connections leads to a better understanding
of the wetland as part of an integrated ecosystem, which in turn
can lead to a formulation of better protection strategies. Much
of the hydrologic activity occurs at the edge of the wetlands.
This has important implications for management of wetland sites
and indicates the need for protection of wetland edges if
functional capacity is to be preserved.
Figure 1. Portions of the Des Moines NW and Grimes 7.5'
quadrangles showing the location of studied wetlands at the Camp
Dodge site. Location A represents the restored wetland and
location B represents the existing wetland.
Figure 2. A model for the development of linked depressions
on the Des Moines Lobe.
Wetland Geomorphology and Stratigraphy
Recent geologic studies on the Des Moines Lobe have identified a complex sequence of sediment that is the product of a stagnant, wasting glacier. As the ice slowly melted, tunnels within the stagnant ice functioned as an internal drainage network. Eventually, these tunnels became clogged with stratified deposits of silt, sand and gravel, and then were covered with less sorted sediments. Shallow, porous sand and gravel bodies now occupy the former glacial tunnels and function as subsurface links between modern-day, semi-closed depressions on the land surface and successively larger surface-drainage routes. These conditions are common on the Des Moines Lobe and are referred to as linked-depression systems. The “linkage” generally is not visible from the land surface, but is present in the subsurface and may have important hydrologic implications. A conceptual model of a linked-depression system with three orders of drainage is presented in the Figure 2. The existing wetland is located in an upland, semi-closed depression, interpreted as a former low-order tunnel. The restored wetland is in an abandoned glacial valley, interpreted as a second-order or higher drainage tunnel.
Restoration Site Stratigraphy
The restoration site has been dated to 9,970 ± 70 RCYBP and
would appear to have been active until recent times. The
stratigraphy is fairly constant over the wetland, and from top to
bottom consists of: dry to wet muck; highly compressed fibric
carbonate-rich peat; muck to peat with abundant shells; silty
clay; and highly organic muck grading to gyttja-like, organic
ooze, overlying outwash sands and gravel (Figure 3). In a few
cores the lower muck unit may be broken into two separate units
separated by another silt unit. At least one of the silt layers
probably corresponds to the middle Holocene warm and dry period,
however further dating is necessary to better explain the
observed sequence. Similar stratigraphy has been seen in other
wetlands located on the Des Moines Lobe.
Figure 3. Selected cores from the restoration site.
The ooze layer at the bottom would suggest that the site may
have been a deep-water ponded site during its early history. The
middle muck units are fibrous in part, indicating fairly
saturated conditions. The upper muck tends to be very degraded
and may reflect more recent conditions, where the filling of the
basin has reduced the influence of groundwater, and the reliance
on precipitation has led to seasonal wetting and drying cycles.
This information is useful in determining restoration goals. If
the site has been a seasonal wetland, then restoration should aim
to re-establish these conditions.
Existing Wetland Stratigraphy
The basin stratigraphy at the existing wetland site consists
of Holocene-age deposits overlying late Wisconsin glacio-fluvial
sand (Figure 4). Within the basin, slope-derived sediments or
sapric muck overlie a clay or silt layer, underlain in turn by
sand and silt with fibric muck beneath, and finally peat over
till. The uppermost basin sediments range from black soil to
mucky organic rich soil, depending on the location of the core in
the wetland.
Figure 4. Selected cores from the existing wetland.
The wetland site stratigraphy can be explained in the context of the regional paleoclimate setting. Radiocarbon dating of the basal peat suggests that around 11,540±60 RCYBP the wetland was functioning with an abundance of water to support aquatic plant life that eventually promoted peat development. The well-sorted sand unit indicates that a period of significant drying occurred in the past (perhaps coinciding with the documented drying period during the mid-Holocene). Since the sand is a clean, well-sorted deposit, the wetland was likely impacted more by wind-blown sediment, and organic accumulation was at a minimum. This conclusion seems plausible since the thin sand layer is found consistently throughout the wetland basin. The uppermost silt and muck units indicate that there have been Holocene climatic fluctuations, which have caused the wetland to be ephemeral, and thus impacted the organic accumulation within the basin and accelerated organic decomposition.
Water-level data for the restoration site shows greater
variability during 1996, as might be expected for a drained site
(Figure 5). There was little or no surface water present at the
restoration site during this year. After the drains were cut in
June 1996 and after late summer-early fall rainfall events, water
levels rose and were above ground surface for much of 1997.
Surface water levels were essentially coincident with groundwater
levels. This implies a greater reliance on groundwater inputs,
however no water budgets have been calculated for the restoration
site.
Figure 5. Water levels at the restored wetland.
Wetland Hydrology
The data from the existing wetland site is consistent with the
ephemeral nature of the basin (Figure 6). Groundwater levels are
often below the surface during late summer, indicating net
seepage out of the wetland. Surface water is often 3-4 feet deep
in the wetland during this time, indicating a slow rate of
seepage out of the basin. Overall flow direction supports the
hypothesis of a linked-depression wetland. Flow is directed to
the subsurface drainage outlet at the southeast edge of the
wetland.
Figure 6. Water levels at the existing wetland.
Note the sharp increase in all well water levels corresponding to a large, mid-summer storm (July 97), followed by a recession. This suggests a greater dependence on precipitation and/or overland flow than groundwater inflows. A water budget developed for the existing wetland corroborates this; precipitation supplies in excess of 80% of the overall wetland budget, followed by groundwater inflow and runoff.
Monitoring of the restored and existing wetland sites continues.
Publications
Water level monitoring of an existing wetland and a restored wetland at Camp Dodge 1996-1999: a summary review: GSB Technical Information Series 43 (Abstract)