The ROMA (River Observatories for Management Applications) Project is designed to provide assessment of the spatial and temporal variations of soil erosion at many scales in the Susquehanna River basin.

Contemporary erosion rates are often affected by land use. Figure 1 shows a heavily impacted farm landscape in Lancaster County, Pa. To evaluate the impact of increased sediment loads on stream processes and ecosystems, it’s important to evaluate contemporary rates in the context of spatial and temporal scale variations. There are several new approaches to comparing contemporary sediment load and sediment yield measurements to the geologic and geomorphic background in drainage basins.

Background, steady state erosion for drainage basins of various sizes measured using in situ 10Be (Bierman and Steig, 1996; Matmon et al., 2003). The erosion rates measured with this technique in the Susquehanna River basin (Reuter et al., 2004) shown on figure 2 range from a few meters/million years (m/Ma) to hundreds of m/Ma. Rates vary significantly for small basin areas (<100 km2) as a function of rock type, basin climate, slope, etc. Rates for larger basins represent integrated contributions from tributaries and tend to correspond to long-term regional erosion rates determined by other methods (Matmon et al., 2003)

We are attempting to construct sediment budgets for the Susquehanna and several of its tributaries. Several techniques for quantifying sediment transport will be discussed. It’s important to recognize that the Holocene geomorphic history involves both climatic and human disturbances. Late Pleistocene to Holocene channel incision in the lower Susquehanna may be important to the pattern of sediment storage in the Susquehanna tributaries. The gorge incision during MIS 2 created a new local base level for tributaries to the Holtwood Gorge. Streams have incised many meters below their pre-glacial thalwegs in response to the gorge incision. Headcuts may be active in the tributaries, but we don’t have time to look at knickpoints on this trip. Due to channel deepening, a large volume of potential sediment storage was created in these tributary valleys in the late Pleistocene and Holocene. As we will see in the course of this trip, storage is most apparent in the low-order tributaries and behind dams. The steep lower reaches of major tributaries do not appear to contain large volumes of sediment relative to their storage capacity. The lack of stored sediment in the higher order tributaries could be due to: continuous or recent removal of those sediments by high discharges or lack of delivery of sediments from lower order tributaries. While we can’t answer that sediment budget question definitively yet, we present evidence that large volumes of sediment were delivered to and from low order tributaries during land use changes.

Meteoric 10Be can be used to assess the relative disturbance and acceleration of erosion from upland soils. Figure 1 shows one of the two cosmogenic isotope pathways that allow comparison of background and accelerated rates of erosion. 10Be is produced by cosmic ray spallation in the atmosphere as well as in situ in minerals at the earth’s surface. [Reusser and Reuter (this volume) will discuss applications of the in situ pathway to date rock exposure by erosion and to trace the slow, background movement of sand-sized quartz from uplands to stream sediments.] Unlike the 14C that cycles through a closed organic pathway (represented by the closed system decay of 14C used to date buried organic material), meteoric 10Be follows an inorganic pathway to soils from the atmosphere via rainfall.

During slow, background basin erosion, soil profiles develop on underlying rocks and sediments. Under humid climates, clay-rich soil profiles accumulate atmospheric 10Be delivered by rainfall (Pavich et al., 1986). Profile distributions, such as measured in a 100,000 year old soil near Chesapeake Bay shown on figure 2, exhibit peak concentrations (atom/g) in clay-rich B-horizons. 10Be is adsorbed and tightly bound at near-neutral pH in soil exhange complexes. Unless disturbed by erosion, inventories of 10Be (atom/cm2) increase through time in clay-rich soils (Pavich et al., 1986; Pavich and Vidic, 1993).

Floodplain storage of eroded soil is also an important process (Trimble, 1999). The legacy of 19th century agriculture is still found in floodplains of eastern streams. 10Be-enriched sediment is stored along Atlantic-slope rivers (Brown et al., 1988). Figure 4 shows “Erosion indices” that compare the export of 10Be on fluvial sediments with input by rainfall. The index is calculated by dividing the average annual export of 10Be on sediment (atom/g x g/cm2/yr) by the average annual input to the basin area (cm-rainfall x atom/cm3/yr). An index value >1 indicates that more 10Be is leaving the basin than can be sustained by rainfall. We interpret that those values indicate a disequilibrium between soil removal and formation. These indices can be compared to contrasting land use land cover histories (Loveland et al., 2002) to assess the relation of accelerated erosion to upland disturbance. On figure 7, there is an apparent correlation of index with land use history in comparing the Susquehanna at Harrisburg with southern Piedmont basins such as the Rappahannock and others further south.

Data have been obtained for Susquehanna tributary basins. The alluvium from instrumented tributaries of the Susquehanna can be used to measure in situ 10Be to determine background erosion rates, and meteoric 10Be on sediments to determine the relative contributions of historic soil erosion among the tributaries.

Bierman, P.R. and Steig, E., 1996, Estimating rates of denudation and sediment transport using cosmogenic abundances in sediment, Earth Surface Processes and Landforms, 21, 125-139.

Brown, L., Pavich, M.J., Hickman, R.E., Klein, J. and Middleton, R., 1988, Erosion of the eastern United States observed with 10Be, Earth Surface processes and Landforms, 13, 441-457.

Gellis, A.C., Pavich, M.J., Bierman, P., Clapp, E., Ellwein, A., and Aby, S., 2003, Modern sediment yield compared to geologic rates of sediment generation in asemi-arid basin, New Mexico-determining the human impact, accepted for publication by Earth Surface Processes and Landforms

Loveland, T.R., T. Sohl, S.V. Stehman, A.L. Gallant, K.L. Sayler, and D.E. Napton. 2002. A strategy for estimating the rates of recent United States land cover changes. Photogrammetric Engineering & Remote Sensing 68(10): 1091-1099.

Matmon, A., Bierman, P.R., Larsen, J., Southworth, S, Pavich, M. and Caffee, M., 2003, Temporally and spatially uniform rates of erosion in the southern Appalachian Great Smoky Mountains, Geology, 31, no.2, 155-158.

Pavich, M.J., Brown, L., Harden, J., Klein, J. and Middleton, R., 1986, 10Be distribution in soils from Merced River tearraces, California, Geochim et Cosmochim Acta, 50, 1727-1735.

Pavich, M.J. and Vidic, N., 1993, Application of paleomagnetic and 10Be analyses to chronostratigraphy of Alpine glacio-fluvial terraces, Sava River Valley, Slovenia, in: Climate Change in Continental Isotopic Records, Geophysical Monograph 78, 263-275.

Reuter, J.M., Bierman, P.R., Pavich, M.J., Gellis, A.C., Larsen, J and Finkel, R.C., 2004, Erosion of the the Susquehanna River Basin: assessing relations between 10Be-derived erosion rates and basin characteristics, Geological Society of America, Abstracts with Programs, 37-3.

Trimble, S.W., 1999, Decreased rates of alluvial sediment storage in the Coon Creek Basin, Wisconsin, 1975-93, Science, 285, 1244-1246.

Valette-Silver, J.N., Brown, L., Pavich, M.J., Klein, j. and Middelton, R., 1986, Detection of erosion events using 10Be profiles: example of the impact of agriculture on soil erosion in the Chesapeake Bay area (USA), Earth and Planetary Science letters, 80, 82-90.

Contact:
Dr. Milan Pavich
River Observatories for Management Applications Project Chief
U.S. Geological Survey
955 National Center
Reston, VA 20192
703-648-6963
mpavich@usgs.gov

Additional information on the ROMA Project is available at http://erg.usgs.gov/rit/.

For more information on USGS activities in the Chesapeake Bay watershed visit chesapeake.usgs.gov.