Using Shuttle Radar Topography to Map Ancient Water Channels in Mesopotamia
Hritz, Carrie, Wilkinson, T. J., Antiquity
The literate and city-based polities of Sumer and Akkad in southern Iraq are widely regarded as among the earliest examples of successful city-based civilisation, and great strides have been made in understanding their archaeology, economy, literature and social conditions. Fundamental to the development of these societies was a network of water channels, which provided irrigation for the cultivation of food crops, and a means of transport of goods from city to city. Irrigation technology and access to a network of channels has therefore been recognised as a key component of the so-called 'Mesopotamian advantage' (Algaze 2001).
The seemingly flat terrain of the Mesopotamian lowlands is drained by the Tigris and Euphrates rivers, which form a system of branching and locally meandering channels. Deposition of fine sand, silt and clay both within and alongside the channels results in the aggradation of levees which gradually raise the rivers until they flow several metres above plain level on low ridges (Buringh 1957). The excavation of canals to divert irrigation water from the main river channels towards fields also results in levees. Formation processes are similar to those of riverine levees, but in addition, channel excavation and cleaning operations result in the up-cast of clean-out banks alongside. As a result of these processes, both natural and artificial channels become raised above plain level to form levees up to 5m or so in height, and up to several kilometres in width. With a gentle gradient of c. 1:10 000 to 1:15 000 towards the head of the Gulf, the plains appear virtually flat, and surface elevation appears to reflect the pattern of levee development. The excavation of new canals as well as the natural branching process of rivers has resulted in an immensely complicated network of natural, artificial and hybrid channels across the plains.
Cuneiform texts have been used to indicate the changing geography of some of the channels over several millennia, and the levees are often visible from the air as low ridges, relict meanders or soil moisture marks, but the complexity of the Mesopotamian landscape makes it extremely difficult to disentangle their history. Despite recent advances (Cole & Gasche 1998; Verhoeven 1998), interpretation has been hampered by the lack of geomorphological data, maps and air photographs. As satellite imagery became increasingly available from the 1970s it has been possible to make general maps of channel systems of the Mesopotamian plains (Adams 1981), and the recent public release of CORONA satellite imagery has resulted in a flurry of new studies on landscapes in both Mesopotamia and beyond (Pournelle 2003; Hritz 2004). The digitisation of topographic maps of the Mesopotamian plains west of Baghdad has provided valuable supplementary micro-topographic data indicating the pattern of river levees and other channel systems (Cole & Gasche 1998). A still higher potential for interpretation has now been provided by the recently released data from the Shuttle Radar Topography Mission (SRTM), which takes the form of digital elevation models (DEM) for virtually the complete globe (Sherratt 2004).
Shuttle Radar Topography Mission
The Shuttle Radar Topography Mission (SRTM), is a joint project between the National Imagery and Mapping Agency (NIMA) and National Aeronautics and Space Administration (NASA), one aim of which is to facilitate military, environmental and economic projects: 'The objective of this project is to produce digital topographic data for 80% of the earth's land surface (all areas between 60 degrees north and 56 degrees south Latitude), with data points located every 1-arc second (approximately 30 meters) on a latitude/longitude grid. The absolute vertical accuracy of the elevation data will be 16 meters (90% confidence) (http//srtm. usgs.gov/Mission/missionsummary.html)'. In the case of the Near East, the available data is for 3-arc second intervals, or 90 metres. …