La doctrina de Pothier

E N D O G E N IC FACTORS F i g u r e 2 . 2 S o m e g e n e ra l f a c t o r s o f f l o o d p l a i n c o n s t r u c t i o n , v a l l e y f i l l a c c u m u l a t i o n a n d t e r r a c e d e v e l o p m e n t ( f r o m B u r r l n , 1 9 8 0 )

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channel and floodplain geometry that involve rainfall, seasonality, intensity and periodicity, as well as runoff, ground cover, sediment calibre and amount. The critical and immediate variables are ground cover, runoff and sediment supply. The ultimate variables are climate and human activity (Butzer, 1970)

Archaeological sites in alluvial settings can be affected by either lateral or vertical accretion. Lateral deposits result from channels changing their location as they shift across non-cohesive bed materials and vertical accretion results from channel overflow and inundation of the adjacent lowlands (ibid.)

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Archaeologists have provided clear evidence that the lateral shift of channels is completely natural and to be expected. The number of archaeological sites in floodplains decreases significantly with age simply because as floodplains are modified by river migration, the earliest sites have the greatest probability of being destroyed

(Schumm, 1977, 132). Examples of archaeological sites being affected by erosion or deposition are found in chapter three, a review of geoarchaeological research in floodplain environments.

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'Schumm and Lichty (1965) believe that distinctions between cause .anji effect in the molding of landforms depend on the span of time [involved and on the size of the geomorphic system under consideration.

the dimensions of time and space change, cause-effect relationships ynaV be obscured or even reversed, and the system itself may be described ■ '.fferently (ibid., 110). During a long period of time a drainage 'Stem or its components can be considered as an open system which is |progressivëly losing potential energy and mass (erosion cycle) , but over Ijsh^rter spans of time self-regulation is important, and components of (the system may be graded or in dynamic equilibrium. During an even ishprter time span a steady state may exist. Therefore, depending on the ^temporal and spatial dimensions of the system under consideration, ^landforms can be considered as either a stage in a cycle of erosion or ?as a system in dynamic equilibrium.

Fluvial activities also affect past population's choice of site location. In addition, the site's preservation and ultimately its discovery, recognition and interpretation are affected by non-cultural processes related to stream flow and flooding. Bettis (1992, 119) states that "the impact of these processes on the archaeological record is usually not considered on a landscape scale." Turnbaugh’s (1978) study of north-central Pennsylvania notes an active preference of prehistoric peoples for terrace locations and implies that terraces hold a certain significance in terms of differential preservation (Turnbaugh, 1978, 604). Campsites, villages and activity stations were selected as part of a cultural formation process pre-determined to some extent by the

population's understanding or appreciation of local stream activity |

(Turnbaugh, 1978, 593). Site selection, in turn, determined to a certain extent differential preservation probabilities among site types. He concludes that

Local aboriginal populations tended to situate their long-term settlements well away from flood-prone areas, while, at the same time, seasonal camps or activity areas which were established during the flood-free part of the year could be set up with little regard for the potential high-water mark (Turnbaugh, 1978, 604).

The differential preservation due to location between sites in flood-prone areas and those on higher ground could be archaeologically mis-interpreted in this instance to place greater

emphasis on village-type settlement sites over seasonal activity I

areas. It is important to note, however, that all the factors affecting preservation rates among site types are likely to be undetermined at present.

Once it is accepted that fluvial activity does affect archaeological resources, it becomes nesessary critically to

acquire, analyse, and interpret archaeological data with the expectation that fluvial action has introduced bias into the record. The bias can affect our ability to interpret site locational preference through differential preservation of sites lying within the floodplain from those without it. Distributional data should also be considered relative to the overall collection or survey strategy of the individuals or agency studying them

(Turnbaugh, 1978, 605).

A bias in the distribution of artifacts within a site can result from fluvial action. If geoarchaeological techniques are applied to these deposits, then the process can be identified, quantified and therefore better understood.

A Typical River Morphology

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It is clear that understanding the impact of natural processes upon the landscape is paramount to accurate archaeological interpretation. What may not be so clear is the basic morphology

of a river system, the observable physical processes of river if

development that are characteristic of all river systems. By first understanding a model of river development, actual river systems

then become easier to comprehend.

A typical river can be divided into three subsystems (Hamblin, 1985, 158). The headwater, tributary or upper reach primarily erodes the landscape and is responsible for collecting the water and sediment and channeling it to the main trunk. The main trunk stream acts as a transportation system between the upper reaches where erosion dominates and the lower reach where deposition dominates. Both accretion and deposition occur in the main trunk portion of a river. The lower end of the river is a dispersing subsystem where most of the sediment is deposited in an alluvial fan or a delta, where the water is dispersed into the ocean.

River channels can also be described as either straight, meandering or braided (See Figure 2.3) (Selley, 1978; Selby, 1985, 268). Alluvium in braided (or sometimes called anastomizing) rivers is characterised by sand and gravel channel deposits excluding any

fine-grained overbank silts and clays. There is generally no S T R A I G H T _{A l t e r n a t e bars.} M E A N D E R I N G Point bars A N A S T O M O S I N G B R A I D E D _{Longitudinal bars,} BRAID ED Lingoid bars

I ■ i J Bars (cove re d m flood s t a g e s )