Authors: David T. Hansen, G. James West, Barbara Simpson, Pat Welch
In developing data for
the analysis of archeological site distributions along the American and
Cosumnes Rivers of California, it was recognized that there is considerable
uncertainty in the spatial representation of site locations. There is
uncertainty in the location and dimensions of known prehistoric sites. It is
suspected that errors in the position of these sites affect the ability to
model the relationship of these sites to key features associated with
prehistoric site record. This study relies on the weights of evidence extension
to ArcView 3.x to generate predictive surfaces. In this extension, prehistoric
sites are represented as point data and weights are calculated for associated
evidential themes of distance to stream channel, landforms, surface elevation,
and slope.
Prehistoric site locations are often associated with natural physical features. Archeologists associate site locations with choices made by prehistoric groups that met their needs at that time. Sites have developed on and in response to changing landscapes.An understanding of the underlying probability distributions of prehistoric sites requires identifying features or variables associated with these site locations. This should improve modeling for prediction of site densities. GIS and automated spatial analysis provides the opportunity to examine large amounts of data to assist in modeling for site prediction. The effectiveness of this modeling is often limited by the quality of information associated with the archeological sites. Characteristics captured in detailed site descriptions often do not match those same characteristics in our digital representation of natural features. This is often compounded by uncertainty in site location.
This study is on prehistoric site distributions for two river systems on the eastern side of the Central Valley of California, the American River and Cosumnes River. The study area is entirely within Sacramento County and extends from the mouth of both river systems up to the lower foothills of the Sierra Nevada. This area has some of the highest recorded site densities for central California. The cultural chronology for these sites extends back 4500 years. However, most occupations date within the last 2500 years. Many of these sites indicate repeated occupations over time.For the purpose of this study, the lower 47.9 kilometers (29.8 miles) of the American River and 58.6 kilometers (36.4 miles) of the Cosumnes River are included. The stream channels of these systems are within 20 kilometers of each other. The overall intent of the program is to characterize prehistoric settlement patterns and densities with these goals:
For this analysis, the weights of evidence extension to ArcView 3.x was used. This extension was developed by the Geological Survey of Canada. It is a Baysian method for evaluating the occurrence of site locations against different data or evidential layers. These data themes reclassified into presence or absence classes can then be combined to produce a predictive surface for site locations. Bonham-Carter provides an description of weights of evidence and its application in GeographicIinformation Systems for Geoscientists. This extension was used to evaluate archeological site distribution for Delta Region of California (Hansen, 2000). As in this earlier study, relationships initially explored using this extension were
Principle data used in this analysis are site locations, stream channels for the river systems, major landforms, and elevation data. For use with weights of evidence, the site locations are treated as point data.
Table 1 identifies the primary sources for these sites for the two river systems.
| Table 1 - Source of Site Locations | ||
| Data Source | American River | Cosumnes River |
| Nilsson 1995 report - polygons | 31 | ---- |
| Kitchen Middens identified by Wier et. al. 1950 | ---- | 23 |
| Point locations from CHRIS database | 16 | 100 |
| Total Number of sites | 47 | 123 |
The best positional and aerial representation of sites is for a portion of the American River form a 1995 report for the Corps of Engineers (Nilsson et. al., 1995). These site locations were mapped in detail on 1:24, 000 scale topographic sheets from field notes. A second group of polygon data was captured from soils mapping for the 1940 to 1941 time period (Weir, et. al., 1950). This mapping was compiled on a planimetric base at a scale of about 1:31, 680. Source maps were scanned and the images rectified to match as best as possible other 1:24,000 scale data layers. The remaining group of data was captured from a tabular database of site locations from the California Historical Resource Information Service Center (CHRIS). All sites were reviewed to remove duplicates, nonetheless some duplication, particularly in the Cosumnes River dataset, may still occur.
Duplication of sites and the actual site location will affect the computation of weights with the weights of evidence extension. Figure 2 shows a portion of a scanned images from the 1950 soil mapping.
In this figure, the white dotted lines and numbers are from the scanned image. The flood plain level in blue is from detailed soil mapping from NRCS published in 1993. Kitchen middens from the 1950 maps carry the symbol 36. In developing the digital representation of kitchen middens, the overall displacement of midden locations was felt to be about 250 meters. This distance is indicated by the light halo around the midden locations. Other points in this figure are from the CHRIS database. While these points are in close proximity to midden locations, they were treated as separate sites for this analysis. In comparing the polygons from the 1:24,000 scale site mapping for the American River study area and the kitchen middens, the following range in polygon size was noted:
The digital source for the river channels is the National Hydrology Database (NHD) with the lines representing the stream channels selected out. For the analysis, these lines represent the current thalweg for the two systems. The NHD layer for this area is the currently available 1: 100,000 scale NHD data. These main channels also serve to define the study areas with a buffer of 500 kilometers as shown in Figure 1. The flow in the American River is considerably larger than the Cosumnes. Flow regimes in this area have large seasonal variations. Most precipitation occurs between October and May during fall and winter months with high flows in winter and spring. Low flows occur during summer and fall. Before development both systems typically had perennial flow. The American has one primary channel and the Cosumnes River has Deer Creek as a secondary channel that parallels the main channel. These trials buffered the lines representing the channels for the American and Cosumnes Rivers for calculation of weights based on proximity to channel. Deer Creek was not buffered for calculation of weights. The initial analysis showed high contrast values for the buffered section of the stream channel. Running the trials separately for the two systems also showed strong contrast values. However, the contrast was much stronger for the American River within a shorter buffer distance of the stream channel. This can be seen in Table 3.
The SSURGO database for Sacramento County from the Natural Resource Conservation Service is the source of both detailed soil information and landforms for this analysis (NRCS). The digital representation of landforms was developed from the SSURGO database based on the description by Roger Parsons of landform and soil relationships contained in the soil survey report for Sacramento County (Turgel, 1993). Figure 3 shows the landforms for the American and Cosumnes Rivers and associated sites. Table 2 contains the extent of major landforms for the study areas and the number of sites.
| Table 2 -- Extent of Landforms in Study Areas | ||||||
| Landform | American River | Cosumnes River | ||||
| Hectare | % Area | # Sites | Hectare | % Area | # Sites | |
| F0 -- Active Flood Plain | 1,524 | 3.4 | 4 | 350 | 0.6 | 5 |
| F1 -- Low flood plain | 5,205 | 11.6 | 7 | 7,715 | 14.8 | 57 |
| F2 -- High flood plain | 2,627 | 5.9 | 11 | 2,550 | 4.9 | 12 |
| Total -- Flood plain level | 9,356 | 20.9 | 22 | 10,615 | 20.3 | 74 |
| T1 -- Low stream terrace | 1,436 | 3.2 | 6 | 400 | 0.6 | 0 |
| T2cd -- Low terrace - channel deposit | 13,075 | 29.3 | 8 | 1,031 | 2.1 | 2 |
| T2ml -- Low terrace - main level | 11,911 | 26.5 | 1 | 23,903 | 46.2 | 28 |
| Total for T2 -- Low terraces | 24,986 | 55.8 | 9 | 24,934 | 48.3 | 30 |
| T3 -- Intermediate terrace | 449 | 1.0 | 5 | 1,390 | 2.4 | 5 |
| T4 -- High terrace and Hills | 8,547 | 19.1 | 5 | 14,588 | 28.4 | 14 |
| Other (Dredge tailings, Urban) | 12 | 0.0 | -- | 0 | 0.0 | -- |
| Total | 44,786 | 100 | 47 | 51,927 | 100 | 123 |
The table shows that while there are differences between the river systems, the extent of major landform groups is comparable. This table also shows the number of prehistoric sites identified with each landform based on overlay of the point data. While this shows some differences between the weights for some landform units, both systems show positive weights and contrast for flood plain levels F0, F1, and F2. It is expected that most sites will be in close proximity to what over time has represented the flood plain. Figures 4 and 5 show the areas associated with sites based on the weights calculated for the landform units. For both river systems, the flood plain units comprise most of the area associated with the sites. This is represented in the landform data by the combined flood plain levels. The first trial with weights of evidence as shown in Figure 1 showed relationships in contrast values based on the flood plain association. The American River also includes some areas on the low stream terrace level and intermediate terrace level. These sites are typically near the contact with the flood plain level.
The
break between the overall flood plain level and the other landforms is fairly
distinct based on the detailed soil data. The break between the currently
active flood plain and the low and high flood plain is very distinct. However,
as is well known with soils data inclusions of other soils and in this case
other landform surfaces can be expected. This will affect the relationship
identified between the point site location and the landform. The break between
the older (Pleistocene) surfaces such as T2, T3 and T4 is not as easily marked
in this gently sloping landscape. Sites on these surfaces are expected to be in
close proximity to the flood plain level even if they were incorrectly located.
Figure 3 shows that the site distribution is frequently near the contact
between the flood plain level and other surfaces.
This includes the currently active flood plain which is expected to migrate across the other.3 flood plain levels. With extraordinary events, it can include the low stream terrace and other low terraces.
For this analysis, lines representing the streams were split into 5 kilometer segments. The buffered area split at these points to generate approximately equal areas. This is shown in Figure 8 with the percentage area of the main flood plain level identified for each section. The area of flood plain ranges from 90 percent at the confluence of the American River with the Sacramento River down to about 2 percent 45 kilometers upstream. The area in each segment of flood plain for the Cosumnes River ranges from about 60 percent down to about 12 percent. The area and width of the flood plain is much more uniform along its length. This does show some clear differences between the two systems.
It is also expected that relative elevation above the nearby water surface has a stronger relationship to site position than absolute elevation above sea level or an elevation datum. The sections of 5 kilometer river segments shown in Figure 8 were used to adjust elevation values back to an overall base level along the stream. Elevation values were maximized on a 150 by 150 meter block basis for calculating weights. Maximum slopes were also generalized to 150 by 150 meter blocks for calculation of weights.
The data layers were reclassified into binary classes of association or of no association. Table 3 shows the data layers and their data values classified as associated with the sites.
| Table 3 - Features Associated with Sites for Combined Weights | ||
| Data Layer | American River | Cosumnes River |
| Landform (polygons) | F0 -- Active Flood Plain | F0 -- Active Flood |
| F1 -- Low Flood Plain | F1 -- Low Flood Plain | |
| F2
-- High Flood Plain |
F2
-- High Flood Plain |
|
| Proximity to Stream Channel (Buffered Distance -- meters) | 100 to 900 | 100 to 2,200 |
| Proximity to Flood Plain Conatact (Buffered Distance - meters ) | 100 to 700 | 100 to 900 |
| Proximity to Low Stream Terrace Contact (Buffered Distance -meters) | 100 to 700 | 100 to 700 |
| Proximity to Low Flood Plain Contact (F1) (meters) | --------- | 100 to 900 |
| Slope Range for Maximum Slope | 1 to 55 | 2 to 28 |
| Adjusted Elevation Range (meters) | 7 to 56 |
2 to 29 |
The layers were combined and produced the probability surfaces shown in Figures 9 and 10 for both streams.
Of the 170 sites in this study, only 31 sites were considered to be well located at a reference scale of 1:24,000. The other 16 sites for the American River were felt to be reasonably well located. Most of the other sites were identified and reported prior to 1960. The accuracy in their position and size is unknown. Based on site descriptions and general positions, it was expected that the sites should show strong relationships to landform position, proximity to stream channel, and location above the active flood plain.
Errors in the position of site locations can affect the generation of predictive surfaces. In weights of evidence, the predictive surface is generated on the area or extent of features associated with or not associated with point events. In this study, the uncertainty in position of prehistoric sites relative to features ranged from15 meters for recently mapped sites to well over 200 meters for kitchen middens. The overall higher values shown for the probability surface of the American River may be do to the better positional accuracy for prehistoric sites along the American River.
Besides running the analysis based on the point test feature being in or out of a polygon theme of landforms, site locations were evaluated against their proximity to landform contacts by buffering that contact. This effectively modeled the uncertainty in the site locations. This reduced the effect of having sites being incorrectly located outside of a landform with which the site is located. In addition, it reduces the effect of inclusions in a polygon feature, if those inclusions also occur nearby.
In an effort to model uncertainty of site position relative to surface elevation and derived slope, these surfaces were generalized. For this area, a generalization to 150 meter square blocks appeared to provide the best relationship to site location. This assisted in developing a predictive surface. Elevation was adjusted back to a simulated water surface at 5 kilometer intervals along the river systems. While this processing did not identify any clear relationships for either elevation or slope, it provides a framework for further analysis of any relationship.
In this study, the spatial uncertainty in the position of archaeological sites was not directly modeled. Besides generating a predictive surface based on other GIS layers containing or not containing a site, this study used proximity of sites to key features, and generalized elevation models. For key features associated with site location, proximity to those features such as stream channels and major landform breaks assisted in developing a predictive surface. This and generalizing elevation and slope data to a level that showed some response to the test sites assisted in developing predictive surfaces for these two study areas. It provides a basis for further work in developing our modeling capability to predict prehistoric site distribution.
Bonham-Carter, Graeme F., Geographic Information Systems for Geoscientists , Pergamon Press, Elsevier Sciences Inc, Tarrytown, New York, 10591-5153, 1994.
Hansen, David T., Visualizing Uncertainty Captured from Source Documents with GRID., Proceedings of the Eighteenth Annual Esri International User Conference, San Diego, CA, July 1998.
Hansen, David T., Describing GIS Applications: Spatial Statistics and Weights of Evidence Extension to ArcView in the Analysis of the Distribution of Archaeology Sites in the Landscape., Proceedings of the Twentieth Annual Esri International User Conference, San Diego, CA, July 2000.
Natural Resource Conservation Service, Soil Survey Geographic (SSURGO) Data Base, U.S. Department of Agriculture, NRCS, National Soil Survey Center, Miscellaneous Publication Number 1527, P.O. Box 6567, Fort Worth TX 76115-0567, January 1995.
Nilsson, E., J. J. Johnson, M. S. Kelly, and S. Flint , Archeological Inventory Report, Lower American River Watershed Investigation, California, Dames & Moore, Inc. for U.S. Army, Corps of Engineers, Sacramento District, Sacramento, 1995.
Parsons, Roger B. ,Geomorphic Surfaces in Soil Survey of Sacramento County California, A. J. Tugel et. al., Natural Resource Conservation Service, Washington DC., 1993.
Tugel, A., et. Al.,Soil Survey of Sacramento County, California,, Natural Resource Conservation Service, Washington, D.C., 1993.
Weir, W. W., Soils of Sacramento County California, University of California, Berkeley, College of Agriculture, Agriculture Experiment Station,April 1950.
West, G. James, David Hansen, and Patrick Welch, A Geographic Information System Based Analysis of the Distribution of Prehistoric Archeological Sites in the Sacramento - San Joaquin River Delta, California, Along the Shores of Time;, Proceedings from and International and Interdisciplinary Conference, Rodger F. Kelly and Gary Franklin, Editors, National Park Service, March 31 to April 3, 1999.
West, G. James, David T. Hansen, Patrick Welch, William Olsen, and Tom Heinzer, A Spatial and Temporal Analysis of Prehistoric Site Distribution in the Lower Reaches of Two Central California Drainages, Paper for prepared for publication, 2002.
The authors would like to acknowledge William Olson for providing information on the regions archeology.