The archaeological record varies with the fluvial style and sedimentation of a river, thus controlling the location, preservation, and recognition of archaeological sites. This project identifies archaeological site patterning and preservation along a fluvial system in Iowa that has been relatively stable since the last glaciation. The Upper Des Moines River in Iowa formed approximately 12,500 yr. cal BP as an englacial channel routing glacial meltwater south along the Des Moines Lobe. During the last glacial retreat, the channel incised forming what is now called the High Terrace (TH). Early post-glacial occupations in Iowa would have had access to this the high terrace and uplands. Thus, the Paleoindian and early to middle Archaic site distributions along the Upper Des Moines River are found in upland and upper terrace locations. The intermediate terrace (TI) formed between 4000 and 1000 yr. cal BP and represents the elevation of the river flood plain during this period. Late Archaic people had access to this surface as well as the TH and uplands. By 1000 yr. cal BP, the Upper Des Moines River had eroded down to bedrock and established a floodplain at the level of the lower terrace. Subsequent erosion produced the modern channel configuration with an established series of dated terraces (High Terrace (TH), Intermediate Terrace (TI), and Low Terrace (TL)). The Woodland period peoples would have had access to the current/modern fluvial landscape. To assess site locations strategies of populations that inhabited the Upper Des Moines River valley in the Holocene, 721 archaeological sites were examined and classified using site reports and artifacts as Paleoindian, Archaic, Woodland, Historic and Unknown. The sites were then mapped and associated with geomorphic features in the valley. As expected, Paleoindian sites and Early to Middle Archaic sites were all located on the upper terrace and uplands because the intermediate and lower terraces had not yet formed. Late Archaic people had access to the floodplain that formed intermediate terrace as well as the uplands and upper terrace. Woodland period sites occurred on high (upland and TH), intermediate (TI), and low (TL and floodplain) elevation landforms along the river. The study demonstrated there was a preference for burials/ceremonial sites being placed at higher elevations (TI and higher) and habitation sites being placed at lower elevations (TL and current floodplain) near the main river channel.

The placement of an unconformable surface within a stratal succession affects the interpreted thickness of units and sequences in contact with that surface. Unit thickness influences the interpretation of basin subsidence, paleotopography, diagenesis, and depositional style. Accurate placement of an unconformity results in true formational thicknesses for formations associated with that unconformity. True thicknesses aid in producing more precise surface to subsurface correlations, isopach maps, and paleogeographic maps. An unconformity may be difficult to identify in the stratal succession due to similar rocks above and below the unconformity and the presence of multiple candidate surfaces. Using statistical discriminant analysis of XRF data, formations bounding an unconformity can be discriminated by elemental composition which results in delineation of the associated unconformity. This discrimination is possible even for rocks that do not have significant differences in provenance if they have experienced distinct diagenetic histories. Elemental differences can be explained by quantity and type of cement. Three discriminant models were created. These models were tested with samples from three formations of similar facies, appearance, and provenance that are all associated with the same regional unconformity. All data, regardless of location, facies, or tectonic feature were used to create the first model. This model achieved moderate success by correctly classifying 80% of known samples. In a second model, data were grouped by facies trends. Separating the data by facies resulted in 94% of known samples being correctly classified. This model was most useful for delineation of an unconformity and discrimination of formations. A third model based solely on location or local tectonic feature produced the best results statistically. 96% of known samples were classified correctly. This third model does not compare locations to each other, thus making it less robust. This last model contributes by adding detail to interpretations made with the facies trend model.

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