South Fork Iowa River Basin Study Area


In the South Fork Iowa River basin (fig. 1), data for the ACT study were collected during water years 2006–09 for surface water, groundwater, overland flow, the unsaturated zone, subsurface drain systems, the atmosphere, and the streambed (table 1). To understand the basin as a whole, surface-water data were collected at the outflow from the South Fork Iowa River basin (site 05451210, fig. 1) to measure flow, major ions, carbon, nutrients, sediment, and pesticides leaving the study area. This site also has been sampled as part of the NAWQA status and trends network and has continuous stream discharge and periodic nutrient, sediment, and pesticide data from samples collected since 1996 (U.S. Geological Survey, 2001, 2008, 2009). Groundwater data also were collected at a number of shallow (water table) wells near site 05451210 (fig. 2).


Surface-water data also were collected at the outflows from two catchments nested within the basin: site 05451080 at the outflow from the headwaters catchment of the South Fork Iowa River near Blairsburg, and site 05451070 at the confluence of three large public subsurface drains that form the origin of the South Fork Iowa River (fig. 1). 


Additional data were collected from the unsaturated zone and groundwater (fig. 2) and from the streambed (fig. 3) for focused studies near site 05451210. These focused studies were designed to investigate the transport of agricultural chemicals through a shallow alluvial aquifer toward the stream and between the shallow aquifer and the stream through the streambed.


At a third focus study area in the headwaters catchment (figs. 4–5), data were collected from the atmosphere, overland flow, subsurface drains, the unsaturated zone, and groundwater to investigate the movement of water and agricultural chemicals through the soil to the water table and through shallow groundwater to the subsurface drain system and eventually to the South Fork Iowa River. 


Precipitation data and other weather parameters were collected near the basin outlet (site WT6, fig. 2); precipitation data, other weather parameters, and air-quality data were collected in the headwaters catchment (site PPT-1, fig. 4).


Environmental Setting


The South Fork Iowa River study basin covers 570 km² in central Iowa, upstream of site 5451210 on the South Fork Iowa River (fig. 1); flow at site 05451210 includes drainage from Tipton Creek. The central area of Iowa has moderate temperatures ranging from a monthly mean of 23.4 °C in July to a monthly mean of –7.7 °C in January. Mean annual precipitation in central Iowa during the past 118 years averaged 83 cm (National Oceanic and Atmospheric Administration, 2002b). However, annual rainfall amounts during the study in 2007 (114 cm) and 2008 (124 cm) were the fourth and second greatest, respectively, on record. Monthly precipitation increases in early spring after snowmelt from less than 5.0 cm in February to more than 12 cm in June (fig. 6). Rain is considered reliable in spring, but decreases throughout the summer. Occasionally, heavy spring rains can delay crop planting. The smallest amount of precipitation occurs during December, January, and February. Evapotranspiration plays a large role in the water budget of the region.


The South Fork Iowa River originates at the confluence of three large public subsurface drains (site 05451070, fig. 1) in Hamilton County, about 2.5 km north of Blairsburg, Iowa. The South Fork flows northeast from the origin for about 7.0 km before flowing in a generally southeasterly direction through Hardin County where it joins the Iowa River near site 05451210, south of Eldora, Iowa (fig. 1). The natural channel of the South Fork Iowa River was extended from western Hardin County to its current origin in eastern Hamilton County with the construction of an artificial drainage ditch in the early 1900s. The ditch was dug to create additional farmland by draining wetlands and potholes. Numerous other drainage ditches join the South Fork Iowa River in the upper half of the watershed.


Land Use


More than 95 percent of the South Fork Iowa River basin is used for agriculture (Tomer and others, 2008b), and more than 85 percent of the agricultural land is used for the production of corn and soybeans (Tomer and others, 2008a). Historically, crops grown in the basin were split nearly equally between corn and soybeans. However, because of the demand for corn-based ethanol, the number or acres used for corn production increased substantially in 2007 (U.S. Department of Agriculture, 2009), and the number of acres used for soybean production decreased proportionally. A broad variety of fertilizers, herbicides, and insecticides are used to enhance production and protect crops. In addition to row crop agriculture, large numbers of confined feeding operations (CAFOs) (Tomer and others, 2008a) are present in the basin. Manure generated from the CAFOs is a major source of nutrients applied in the basin.


The South Fork Iowa River basin is sparsely populated. Most of the population lives in rural areas and in three small communities of less than 500 residents. Less than 8,000 people live in the South Fork Iowa River basin (U.S. Census Bureau, 2000a, 2000b). Water for domestic and animal use is almost entirely from ground water. The amount of water used for farm animals probably is several times greater than the amount used for domestic purposes in the basin.


Physiography, Geology, and Soils


Physiographically, the South Fork Iowa River basin is completely contained within the Des Moines Lobe, an area that was glaciated, leaving behind a till plain, lateral moraines, and major glacial drainageways (fig. 7; Quade and others, 2000; Quade and Giglierona, 2006). Glacial and fluvial deposits that range from 20 to more than 75 m in thickness overlie Mississippian limestone and dolomite bedrock formations (fig. 8). Glacial deposits primarily consist of till with interbedded sand and gravel lenses. Sand and gravel eroded from the glacial deposits by streams are located adjacent to the South Fork and its tributaries in the lower portion of the basin.


Soils were developed under long-grass prairies on the glacial till deposits and are highly productive. Soils generally grade from well-drained oxic soils on the hills and ridgetops to somewhat poorly to poorly drained soils on the side slopes. Depressions in the landscape have hypoxic, poorly drained soils that developed under prairie potholes and wetlands. Drainage has been improved in large parts of the basin by installation of subsurface drains and open surface ditches (Tomer and others, 2008b). Available water capacity of the soil is high, and organic matter content is high to moderate. These soils are well suited to row crops, such as corn and soybeans. 


Unconsolidated and bedrock aquifers underlie the South Fork Iowa River basin. Unconsolidated aquifers consist of fluvial sand and gravel deposits located adjacent to the natural channels of the streams and glacial deposits of sand and gravel found scattered throughout the till. Unconsolidated aquifers are a minor source of water for domestic and agricultural supplies. Sandstone bedrock units of the Mississippian aquifer system that underlies from 15 to more than 30 m of unconsolidated material (Olcott, 1992) is the major source of water for domestic and agricultural purposes.


Hydrology


Much of the flow in the South Fork Iowa River originates from subsurface agricultural drain flow. During extreme events, when rainfall exceeds the infiltration capacity of the soil, most water at the origin of the South Fork Iowa River (site 5451070, fig. 1) probably originates from runoff to surface ditches and the surface inlets of subsurface drain networks. Downstream at site 05451080 (fig. 1), the average annual discharge (2006–08) of about 0.5 m³/s primarily is from subsurface drains. At the basin outflow (site 05451210, fig. 1), the average annual discharge is about 5.2 m³/s. 


In the upper basin, at both sites 05451070 and 05451080, the South Fork Iowa River has periods of low flow. At base flow, water contributing to flow at site 05451080 originates primarily from subsurface drain discharge with little or no groundwater seepage directly into the man-made channel. In contrast, discharge during base-flow conditions at site 5451210 is a combination of inflow from subsurface drains and from groundwater seepage directly into the river where coarse-grained, alluvial deposits are present adjacent to the river.


Major modifications have been made to the natural hydrology of the South Fork Iowa River basin in support of agriculture. A network of surface drainage ditches were dug and subsurface drains were installed in the early part of the 1900s to facilitate the drainage of extensive wetland areas to allow production of crops. Installation of subsurface drains has continued to the present (2011). In many areas, excavated materials were deposited as berms adjacent to ditches. A berm commonly dams surface runoff directly into the ditch, and culverts typically were installed through the berms and ditch banks to allow runoff to flow into the ditch. The enhanced drainage permits better timing of seasonal cultivation, lowers the cost of cultivation, and improves seed germination. 


Floods in the South Fork Iowa River generally are caused by spring rains on frozen or saturated soils, by heavy summer thunderstorms, and by fall rains after harvest. The worst recorded flood in the study basin, with an estimated occurrence probability of between 0.2 and 1 percent in the headwaters catchment to between 2 and 4 percent in the South Fork Iowa River basin, occurred in June 2008 (Buchmiller and Eash, 2010). Periodic droughts in central Iowa have lasted from weeks to years. The lack of water during droughts negatively affects the human population, livestock, and agriculture. The worst recorded drought in north-central Iowa—with a recurrence interval of 25 to 60 years—extended from 1933 through 1936. The smallest 7-day minimum streamflow recorded at site 05451210 was 0.05 m³/s, and occurred during September 2000 (U.S. Geological Survey, 2001). 


The site 05451070 catchment, nested within the site 05451080 catchment, has an estimated area of 7.0 km² (fig. 3). The area of the catchment is estimated because the extent of the subsurface drainage network in unknown. Subsurface drainage accounts for most of the flow at site 05451070. This flow is a combination of water that has infiltrated the soil and subsequently moved to horizontal drains and water that has entered the drains through vertical inlets in fields and roadway ditches. Overland flow contributes water directly to the stream only during extremely intense rain events. 


Site 423135093373301 (fig. 4) is 3.2 km downstream of site 05451070. This site is situated at the edge of the stream that receives surface runoff (overland flow) from a 90-ha catchment. Runoff from light to moderate rains from the western two-thirds of this small catchment flows into the South Fork Iowa River through subsurface drains. Runoff from the eastern one–third of this small catchment flows overland and enters the South Fork Iowa River through a 0.46 m culvert installed in the streambank. During extreme rain events, when subsurface drain capacity is exceeded, water can flow overland from the entire 90-ha catchment into the streambank culvert.


Site 05451080 is located about 7.6 km downstream of site 05451070 (fig. 4). The catchment for this site has an area of 31 km². Flow at site 05451080 is a mixture of water from precipitation, subsurface drains, groundwater discharge, and overland flow. 


Two subsurface drains were included in a focus study (fig. 4). The outflow of one drain (423232093351801, site TD-2, 20-cm diameter) is located on the western side of the stream, approximately 0.19 km downstream of site 05451080. The outflow of the second drain (423230093351501, site TD-3, 40-cm diameter) is located on the western side of the stream, approximately 0.26 km downstream of site 05451080. The locations and extents of the subsurface drain networks associated with these two drain outflows are partially unknown, but generally the subsurface drains were installed in the topographically low areas. The area drained through site TD-2 may be from 20 to 25 ha. The area drained by TD-3 may be twice as large—greater than 50 ha. The subsurface drains discharging at TD-3 has at least one vertical inlet in a nearby field. 


Data Collection


At site 05451210, water samples were collected monthly to weekly from 2006 to 2009. A continuous water-quality monitor measured temperature, specific conductance, pH, and dissolved oxygen during ice-free periods from April into November.


Continuous streamflow at site 05451070 was estimated from stage from October 2007 through November 2008. Water-quality data were obtained from samples collected approximately monthly during the winter and early spring and weekly during the late spring and summer 2007 and monthly in 2008.


Stream stage was recorded continuously from 2006 to 2009 at a gaging station at site 05451080. Water samples at this site were collected monthly to weekly from 2006 to 2009. Supplemental samples were collected periodically during storm events by an autosampler triggered by rising stream levels. A continuous water-quality monitor measured temperature, specific conductance, pH, and dissolved oxygen during ice-free periods from April into November. 


At site 423135093373301, flow was measured in the streambank culvert with a stage discharge relation and with an acoustic flow meter. Water samples were collected manually and with an autosampler triggered by rising water levels. The water temperature and specific conductance of the runoff was measured with a continuous water-quality monitor. Flow and water-quality data were collected at this site from April through September 2008.


For both subsurface drains (sites TD-2 and TD-3; fig. 4), discharge measurements were made with an acoustic flow meter. Water-quality samples were manually collected periodically and storm samples were collected by an autosampler triggered by decreasing specific conductance values that are indicative of increasing subsurface flow after the onset of rain. Continuous water-quality monitors measured temperature and specific conductance of the subsurface discharge. 


Near the outlet from the headwaters catchment (fig. 5), data were collected from nests of suction lysimeters, soil‑moisture probes, and shallow wells that were installed in transects. The transects were perpendicular to one of the subsurface drains and roughly parallel to the channel of the South Fork Iowa River. One transect was located in the cropped field about 65 m from the stream and the other was located in a grassy buffer strip about 30 m from the stream. The lysimeters were placed about 0.91 m below land surface to collect water samples in the unsaturated zone. The soil moisture probes were located at approximately 0.3 and 0.6 m below land surface to measure soil moisture. Wells were located at approximately 1.2, 3.0, and 4.3 m below land surface to monitor water levels and the water quality of the saturated zone. 


Rain samples were collected weekly for analysis of selected pesticides at site PPT-1, just west of site 05451080 (fig. 4). Samples were collected in 2007 and 2008 from April through September using a modified automatic wet‑dry precipitation collector. Collected rainwater was funneled into a glass carboy inside a small refrigeration unit located beneath the deposition collector. Refrigeration was used to help minimize evaporative losses of the water and to reduce volatilization losses and biotic and abiotic degradation of the nutrients and pesticides during the sample collection period. Samples were a composite of precipitation events that occurred during a 7-day period. Weekly air samples for selected pesticides and total suspended particulates were collected with calibrated vacuum air samplers that integrated over the 7-day period of collection.


At the downstream focus study area near the basin outlet, streambed temperatures were measured at two depths in a 450-m reach of the South Fork Iowa River upstream of site 05451210 in the summer of 2007 and 2008 to map areas where groundwater was seeping into the river or stream water was moving into the underlying aquifer. Streambed temperatures were measured at shallow (15 cm) and deep (50 cm) points at 3-m increments at 40 cross sections in the stream (fig. 9). Cool temperatures in the streambed—similar to those in the shallow alluvial aquifer—were inferred to mean that water was moving from the aquifer to the stream and warm streambed temperatures were inferred to mean that either stream water was present or there was no movement of water between aquifer and stream. To better understand temporal changes in movement of water between the river and alluvial aquifer, continuous monitors were used to measure water temperature and specific conductance in a streamside piezometer for comparison with water temperature and specific conductance in the South Fork Iowa River. Water temperature and specific conductance in water about a meter below the streambed was measured continuously from May 2008 through March 2009 at piezometer TR1-1A at the edge of the river about 160 m upstream of the New Providence gage. 


Just upstream of the basin outlet (fig. 3), detailed data were collected at several locations in 2007 and 2008 in and near the stream. Temporary drive-point piezometers were installed to collect subsurface water samples and measure the hydraulic gradient between the stream and the subsurface. Each location consisted of a three by three array of piezometers, generally located 2 m apart. Multiple arrays were measured to document any differences due to stream conditions. Arrays were located in a straight reach (TR1), a riffle (TR2), an inside bend (TR3), and an outside bend (TR4) of the river. The hydraulic gradient and water samples were collected from 0.3, 0.5, and 1.0 m below the sediment-water interface in the channel. 


First posted February 16, 2012