Clear Creek and Critical Zone Research


Critical Zone Research

The critical zone of the earth extends from the top of the tree canopy to the bottom of our drinking water aquifers, which is where we humans, and all terrestrial life on earth thrive. To address the research needs within the critical zone, groups of scientists have formed observatories to monitor and study the key processes that shape the critical zone and all life in it. One such network is the Critical Zone Exploration Network (CZEN), in which Clear Creek is currently an Affiliated US Field Site. Additionally, Clear Creek is an International Critical Zone Observatory in the Soil Transformations in European Catchments (SoilTrEC). The aims of SoilTrEC are to address the priority research areas identified in the European Union Soil Thematic Strategy and to provide leadership for a global network of Critical Zone Observatories (CZO) committed to soil research.


Clear Creek Observatory

Site Description

The Clear Creek, IA watershed (HUC-10: 0708020904) covers approximately 270 km2 in east-central Iowa and empties directly into the Iowa River near Coralville, IA (Figure 1).  Clear Creek is representative of most U.S. Midwestern watersheds regarding land use (predominantly agricultural), soil type/order (Alfisols and Mollisols), and climate (humid-continental).  The combination of extensive agricultural activities, increased urbanization, highly erodible soils, and a wet climate on the steep slopes within the watershed has influenced the erosion processes therein (Abaci and Papanicolaou, 2009).

Clear Creek Watershed

Aerial Image of Clear Creek, IA from the 2009 National Agricultural Imagery Program. The blue lines represent the streams.

Land Use

Since European settlement, over 80% of the watershed has been converted from a prairie and forested area to row-crop agriculture and pastures, which is typical for the region. Currently, the dominant rotation in the watershed is corn-soybean (60% of the watershed) and the two crops are in roughly equal proportions throughout the watershed. Pasture and other grasslands cover 23% of the watershed with 10% covered by forest and 7% in urban areas (Rayburn and Schulte, 2009).

A detailed history of land use (e.g., Figure 2; Rayburn and Schulte, 2009) and management practices (e.g., Abaci and Papanicolaou, 2009; Wilson, Papanicolaou, and Abaci, 2009) exists for the watershed. Major land use changes that have taken place in Clear Creek since 1940 include the following (Rayburn and Schulte, 2009):
1. Increases in urban cover (+1743.0 ha) and housing density (2.6 houses/km2 in 1940 to 14.1 houses/km2 in 2002).
2. Increases in dense forest cover (+618.9 ha) and mean forest patch area (+3.7 ha).
3. Decreases in total area (−2773.4 ha) and mean patch area (−4.9 ha) of row crops.

Land cover of the Clear Creek watershed in east-central Iowa, USA, for (a) 1940 and (b) 2002 (Rayburn and Schulte, 2009)

Land cover of the Clear Creek watershed in east-central Iowa, USA, for (a) 1940 and (b) 2002 (Rayburn and Schulte, 2009)

The landscape was converted from native prairie to row crop agriculture around the 1930s. Since then, different predominant agricultural management strategies have been utilized. A 5-year crop rotation was utilized from 1931-1950, consisting of corn, oats, and alfalfa. In years 1 and 2 of the rotation, corn was planted with tillage using the moldboard plow in both spring and fall. In year 3, oats and alfalfa were planted simultaneously. In years 4 and 5, the alfalfa was cut and baled as hay in both June and September. In 1951, manure applications were replaced with inorganic fertilizers, while all other practices remained the same. This period lasted until 1975.

Soybeans were introduced in a 3-year rotation (CCB) in 1976 with a higher intensity of cultivation from advancements in mechanized machinery. The current rotations in Clear Creek began in 1991 as 2-year rotations of corn and soybeans with the use of conservation tillage practices (reduced and no-till). These rotations are identified as the following: spring till corn – no till bean; fall till corn– no till bean; and no till corn–fall till bean. The spring tills were a reduced tillage performed in April with a field cultivator, whereas the fall tills were performed in October with a chisel plow and were deeper. A shift is beginning in the watershed as many farmers are beginning to plant consecutive years of corn.


The watershed is situated in the Southern Iowa Drift Plain (Figure 3), which is comprised of glacial deposits broken up by a multitude of small creeks and streams. This area was last glaciated during the pre-Illinoian period, and has been greatly dissected due to erosional processes during periods of interglaciation (Prior, 1991). Windblown Peorian loess deposits covered the entire Southern Iowa between 14,000 and 25,000 years ago to depths ranging from 5 to 30 feet (Ruhe, 1969; Hallberg et al., 1978).

Erosion triggered by flowing water and enhanced by agricultural activities in the watershed has re-worked the landscape into series of rolling hills and valleys. The current surface of the Southern Iowa Drift Plain has patches of the older pre-Illinoian till deposits that have been buried by loess but are now exposed due to erosion (Ruhe et al., 1967; Prior, 1991).

Major Landforms.  In Iowa there are seven major landform with each one represented by a different color above.  Each landform is characterized by a unique principal soil association. From Prior (1971)

Major Landforms. In Iowa there are seven major landform with each one represented by a different color above. Each landform is characterized by a unique principal soil association. From Prior (1971)

The Loess derived soils (Figure 4) are some of the most productive agricultural lands due to their strong soil structure and nutrient availability (Jones et al., 1967). The Peorian Loess contains a mixture of sediments from glacier-derived silt and clay and nonglaciogenic sources (Bettis et al., 2003). Being loess-derived, the soils are relatively homogeneous, consist of predominantly silt-sized particles, and highly erodible. The dominant surface soil texture within Clear Creek is silty clay loam. Moving downstream, the dominant soil texture changes from a silty-clay loam in the headwaters to a silty-loam near the mouth.

Mollisols are the dominant soil order found in the watershed with Alfisols, Inceptisols and Entisols also present but to lesser extents. The most common soil associations in Clear Creek are the Tama-Downs, Fayette-Downs, and Colo-Nevin-Nodaway associations (Dideriksen et al., 2007).

The Tama-Downs and Fayette-Downs upland soils are both well drained soils found on a biosequence. Tama soils formed under prairie while Downs formed under savannah. Fayette and Downs soils are found on similar landscape positions, but the Fayette formed under deciduous forest.

The Colo-Nevin-Nodaway is an association split according to drainage class. All of these soils formed in either stream terraces or flood plains, but the drainage class ranges from poorly drained to moderately well drained. Floodplains in Clear Creek are alluvium-derived soils comprised of mostly the Colo series (Highland and Dideriksen, 1967). Soils in the Colo series are poorly drained (Highland and Dideriksen, 1967).

Clear Creek Soil Series

Soil series map of the Clear Creek watershed from the Iowa Soil Properties and Interpretations Database maintained by the Iowa Cooperative Soil Survey and Iowa Geological Survey.


Due to the mid-continental location of Iowa, the climate in Clear Creek is characterized by hot summers, cold winters, and wet springs (Highland and Dideriksen, 1967). Summer months are influenced by warm, humid air masses from the Gulf of Mexico. Daily high temperatures reach an average July maximum of 29o C. Dry Canadian air masses dominate the winter months. Daily low temperatures reach an average minimum of -13o C in February.

Average annual precipitation is approximately 889 mm/yr with convective thunderstorms prominent in the summer and snowfall in the winter, which averages 762 mm annually. Monthly, precipitation averages from approximately 1.0 mm/day between December and February to almost 6.0 mm/day in June. The majority of streamflow occurs during spring and summer, with peak monthly streamflow in May and June. This data was compiled from National Weather Service and Iowa Mesonet websites.


Clear Creek is approximately 40 river km long with a sinuosity between 1.27 and 1.49. However, the stream has experienced widespread channelization and modification through the construction of drainage systems to facilitate water movement through the system (Rayburn and Schulte, 2009). The average slope of the main stream channel from the headwaters to the mouth is 0.001 (Loperfido et al., 2009). The average bank height is 5.8 m with an average bank angle of 47o.

The United States Geological Survey (USGS) operates stream gages near Oxford, IA (#05454220) and Coralville, IA (#05454300), which is near the outlet. At the outlet, the average water discharge and sediment loading through this reach is 2.05 m3/s (daily or 64.6 x 106 m3/yr) and 0.76 kg/s (daily or 23.9 x 103 tons/yr), respectively (Abaci and Papanicolaou, 2009). The flow regime is less flashy here than at the headwater reach, with a Richards-Baker Flashiness Index of 0.36, placing it the lower half of flashy streams for watersheds of this size (Baker et al., 2004).


Abaci, O., and A.N. Papanicolaou. 2009. Long-term effects of management practices on water-driven soil erosion in an intense agricultural sub-watershed: monitoring and modeling. Hydrological Processes. 23:2818-2837.

Baker, D.B., R.P. Richards, T.T. Loftus, and J.W. Kramer. 2004. A new flashiness index: Characteristics and applications to Midwestern rivers and streams. Journal of the American Water Resources Association. 40(2):503-522.

Bettis, III, E.A., D.R. Muhs, H.M. Roberts, and A.G. Wintle. 2003. Last glacial loess in the conterminous USA. Quaternary Science Review. 22:1907-1946.

Dideriksen, R.O., M.R. LaVan, K.K. Norwood, S.R. Steckly, and J.E. Steele. 2007. Soil survey of Iowa County, Iowa. USDA-NRCS.

Hallberg, G.R., N.C. Wollenhaupt, and J.T. Wickham. 1978. The Iowan erosion surface: An old story, an important lesson, and some new wrinkles. In: R. Anderson (ed.) 42nd Annu. Tri-State Geol. Field Conf. Guidebook. p. 2-1 to 2-94.

Highland, J. D. and R. I. Dideriksen. 1967. Soil survey of Iowa County, Iowa. U.S. Department of Agriculture, Soil Conservation Service, in cooperation with the Iowa Agricultural Experiment Station.

Jones, R.L., B.W. Ray, J.B. Fehrenbacher, et al. 1967. Mineralogical and chemical characteristics of soils in loess overlying shale in northwestern Illinois. Soil Science Society of America Proceedings. 31:800-804.

Loperfido, J.V., C.L. Just, and J.L. Schnoor. 2009. High-frequency diel dissolved Oxygen stream data modeled for variable temperature and scale. Journal of Environmental Engineering-ASCE. 135(12):1250-1256.

Prior, J.C. 1991. Landforms of Iowa. University of Iowa Press for the Iowa Department of Natural Resources, Iowa City, IA.

Rayburn, A.R., and L.A. Schulte. 2009. Landscape change in an agricultural watershed in the US Midwest. Landscape and Urban Planning. 93(2):132-141.

Ruhe, R.V. 1969. Quaternary landscapes in Iowa. Iowa State University Press, Ames.

Ruhe, R.V., R.B. Daniels, and J.G. Cady. 1967. Landscape evolution and soil formation in Southwestern Iowa. Tech. Bull. 1349. USDA-SCS, US Government Printing Office, Washington DC, USA.

Wilson, C.G., A.N. Papanicolaou, and O. Abaci. 2009. SOM dynamics and erosion in an agricultural test field of the Clear Creek, IA watershed. Hydrology and Earth System Sciences Discussions. 6:1581-1619.