Canals and Aqueducts in the Ancient World
People of the ancient world understood that geophysical and climatic anomalies could alter the environments that permitted the growth of comestible agricultural resources for urban and rural populations. When their technical capability proved adequate, they were able to modify water supply systems to sustain agricultural productivity through times of environmental change. When technological solutions or adaptations to other resources were not possible, societal transformation and/or collapse followed, leaving archaeological remains that now testify to the lack of appropriate technology, management, or manpower to overcome the deteriorating resource base. Water for urban and agricultural use is vital to sustainability. When the collapse of agricultural systems is manifest in the archaeological record, remains of canals, aqueducts, water storage, and transport systems provide vital geoarchaeological clues detailing how and why failure occurred. These clues often point to long-term drought that limited water availability for farming, floods that incurred changes in agricultural landscapes through soil erosion or aggradation, seismic/tectonic effects that disrupted canal and aqueduct systems, river downcutting (rejuvenation) that stranded canal inlets, and aeolian soil transport that led to landscape inflation or deflation processes. The influence of these geoarchaeological processes on water supply and distribution systems is basic to (1) understanding the fate of ancient sites and cultures and (2) interpreting the processes of societal collapse and transformation.
Geoarchaeological effects on canals in ancient South America
The Middle Horizon (300 BCE–1100 CE) site of Tiwanaku (Bolivia) also underwent a collapse due to long-term drought in the tenth to eleventh centuries. A dropping water table and declining spring flow ultimately stranded 100,000 km2 of raised field systems adjacent to Lake Titicaca that supported agriculture for the Tiwanaku capital city (Ortloff and Kolata, 1993); no 14C dates indicating occupation after 1100 CE are recorded for the capital city or satellite centers.
Ancient South American societies experienced a variety of climatic effects that induced geophysical landscape changes affecting their agricultural and marine resource base. The disappearance and/or transformation of major societies, and their resurrection under different social structures in different areas when drought conditions relaxed in the twelfth to thirteenth centuries CE, depended largely upon exploitation of more sustainable agro-systems that benefitted from increasing water resources. Colonies and satellite settlements characterized many major Andean societies (Murra, 1962). This strategy expanded agriculture into different ecological zones and applied different farming techniques, employing local water resources to lessen the dependence upon a single agro-system type. This approach proved valuable in diversifying the agricultural resource base of Andean societies.
Geoarchaeological effects on societies of the ancient Middle East
While climate change and severe weather events characterize coastal and highland societies of ancient (and modern) South America due to ENSO El Niño and La Niña drought and flood effects, ancient (and modern) Mediterranean societies had the advantage of milder climate and fewer significant weather fluctuations. For the most part, Roman, Greek, and Levantine civilizations experienced fewer climate-related environmental challenges than their South American counterparts. While drought and flood events certainly occurred, colonies and captive areas under central state authority that encompassed vastly different ecological zones guaranteed a resource base available through trade and tribute to sustain the large populations of capital cities. Under stable climate/weather conditions, canal and aqueduct construction exhibited a degree of permanence that reflected the monumental labor input dedicated to their construction. This is in contrast to ancient South American societies whose survival and continuity depended upon water transport and agricultural systems that had to be modified to accommodate changing ecological conditions.
At Ephesus, canal and aqueduct systems leading from mountain spring sources distributed water into the urban core through complex pipeline systems that supplied water for 250,000 inhabitants at 150 gal/day/person, including baths, fountains, nymphea, latrines, elite housing compounds, public buildings, a coliseum, gymnasia, and a theater, all of which required a continuously running water supply (Figure 5). The nearby Temple of Artemis, originally ∼3 km inland from the Mediterranean shoreline, was served by multiple underground pipelines of different designs for different ceremonial uses (Ortloff, 2009). This site, originating around 800 BCE, became inundated after many centuries of operation as a result of coastal subsidence and progradation, uplift of inland mountains, and sediment deposits interfering with springs that supplied water to the temple. Here, the geophysical effects were so gradual and subtle that compensatory structural engineering considerations made in advance of construction were apparently not a major concern.
The site of Petra in Jordan (Bourbon, 1999; Taylor, 2001; Guzzo and Schneider, 2002) is a further example of the creative use of intermittent water supplies from rainfall and springs to maintain city activities over centuries. Spring systems within tens of kilometers of Petra provided water to reservoirs from which terracotta pipes guided the flow to inner urban precincts for agoras, fountains, theater, water gardens, temples, public buildings, and domestic housing. Piping systems of different hydraulic designs were necessary given how distant these springs were from distribution hubs. Known from ancient times, and verifiable from modern computer calculations, is that a linear increase in supply hydraulic head does not result in a corresponding linear flow rate increase in long pipelines (Ortloff, 2009) due to nonlinear, cumulative water-internal pipe wall friction effects. This design constraint, together with how landscape-governed slope variations place constraints on pipe flow rate, results in a catalogue of hydraulic designs (Ortloff and Kassinos, 2003; Ortloff, 2009) that were utilized at Petra for the Siq, Jebel el Kubtha, and Zurraba water supply systems. Such urban core water supply systems possessing different hydraulic solutions for different geophysical constraints demonstrate that the ancient engineers possessed a wide knowledge base, approaching in many cases that of modern hydraulic design practice. Rainfall catchment basins and reservoirs provided additional water supplies and limited runoff into the urban center; some 250 such basins have been located in the mountainous areas surrounding Petra. The predictability of rainfall periods in this area of Jordan was well understood in antiquity and served to provide city reservoirs with water through many catchment basins and springs. Defensive water diversion channels and dams limited water damage to the urban core of Petra; here knowledge of water control was key to the permanence of the city for many centuries.
For the sites mentioned, the permanence of construction of fixed water supply elements (canals, pipelines, aqueducts, and reservoirs serving city and agricultural systems) indicates that geophysical threats were minimal outside of occasional, but reparable, earthquake damage. Thus, with regard to water supply systems, the advantage of the Mediterranean world with its stable climate and weather norms is apparent compared to New World cities and settlements.
Geophysical effects on the water systems of the Khmer Kingdom city of Angkor
Southwestern Native American societies: geophysics of canals and aqueducts
Over geologic epochs spanning millions of years, the Colorado Plateau has been etched by the deepening and headward extension of innumerable small valleys opened during periods of intermittent heavy rainfall. These valleys are characterized by floodplain incision from rain runoff producing areas of unconsolidated sediment deposits within the valleys that limit water control for irrigation agriculture (Longwell and Flint, 1962; Cooke and Reeves, 1976). Heavy rains lead to sediment deposition over bedrock, creating arable land for irrigation agriculture but not in areas prone to periodic erosion. Farther south in the Basin and Range country, deeper alluvial valleys containing sandy desert soils limit agricultural productivity due to limited moisture retention, as well as climate/weather conditions characterized by high desert temperatures and more frequent drought conditions. Across the Southwest, many alluvial valleys are prone to stream entrenchment (arroyo cutting) that lowers water tables and restricts the amount of arable land that can be irrigated (Cooke and Reeves, 1976). Thus, as a result of heavy flood runoff and periodic droughts, agriculture was limited by both climate and geomorphic processes that placed constraints on water control.
Yet despite difficulties with unstable farming terrains in these geographic zones, Spanish settlers coming into the area post-1540 CE found land being productively farmed by indigenous peoples (Doolittle, 2000) who ingeniously modified the landscape to capture and store intermittent rainfall and snowmelt to sustain crops (Anschuetz, 2001, 2006; Plog, 2008). On the Colorado Plateau, for example, most of the agriculture noted by the Spanish was floodwater farmed. Periods of drought that deteriorated grasslands and amplified erosion during heavy rainstorms, together with periods of light, but more frequent, rains caused continuous infilling of valleys with alluvium. Farming required adaptive responses to prehistoric climate variability influenced by El Niño and La Niña rainfall and drought periods, respectively (Dean and Robinson, 1977; Fish and Fish, 1984; Doolittle, 1992; Damp et al., 2002). Several of the major prehistoric (pre-Columbian) Indian societies of the Colorado Plateau (e.g., Anasazi and Mogollon) and southern Basin and Range areas outside of the Plateau (e.g., Hohokam and Patayan) farmed floodplains watered by melting winter snow and summer rains. Additional hill-slope terracing was used to stabilize planting surfaces, and flood diversion dams were built to limit erosional/depositional effects on field plots (Doolittle, 1992; Lightfoot and Eddy, 1995; Doolittle, 2000; Anschuetz, 2001).
Diverse technical innovations founded upon highly evolved indigenous cultural knowledge allowed for successful crop production in distinct geomorphic settings (Woosley, 1980; Doolittle, 1992, 2000). For example, the prehistoric Puebloans (Anasazi) located on Mesa Verde in Colorado (Ferguson, 1996) constructed a series of four reservoirs (Box Elder, Morefield, Far View, and Sagebrush) that were operational from 750 to 1180 CE and captured rainfall runoff to redistribute water for agricultural and domestic use (Leeper, 1986; Wilshusen et al., 1997; Wright, 2003). Ethnographically, the Tewa of north-central New Mexico employed bermed terraces to capture rainfall, together with stone-lined transport canals, dams, and spreaders to capture (or divert) runoff. The Tewa could exploit a combination of direct precipitation, intermittent runoff, groundwater, and canal extraction from springs and rivers to water their fields (Doolittle, 1992, 2000; Anschuetz, 2001, 2006). Stone-mulched and stone-bordered sunken pits were also used by Pueblo society to trap precipitation. Anschuetz (2001, 2006) describes anecdotal evidence that winter snow was rolled into balls and deposited in these pits in order to store water and amplify soil moisture for later agricultural use. The Hohokam (600–1350 CE) of south-central Arizona utilized extensive canal networks drawn from rivers and springs to irrigate vast field areas. The Salt River Valley contained as many as 400 km of main and distribution canals (Howard, 1987; Doolittle, 2000; Plog, 2008) with the Gila and Verde Valleys containing yet more irrigation canals estimated to be on the order of 600 km in cumulative length.
South-central Arizona employed the greatest extent of canal irrigation compared to all other southwestern indigenous societies. Aerial photography of these prehistoric canal and field system complexes taken 80 years ago (Judd, 1930) has proven indispensable in discovering and documenting trace canal, and field system remains now obliterated by erosion, sediment deposition overlays, and modern agriculture proceeding from urban expansion. Canal water transport technologies practiced by the Pima (Akimel O’odham) along the Gila River involved long, low-slope, open channels that supplied field systems. While similar water control systems were used elsewhere in the Southwest, canals originating from smaller river tributaries to major rivers were a preferred strategy due to easy water control practices. Other agricultural practices depended upon floodwater farming, especially in areas lacking large, perennial rivers. For example, the Papago (Tohono O’odham) were known for their ak chin (floodwater) farming along ephemeral streams, while the Navajo and Hopi planted fields in drainage areas where floods and runoff occurred during heavy rains (Plog, 2008). Further innovative indigenous agricultural strategies practiced in the Southwest are summarized by Doolittle (2000) and Plog (2008).
Modern scientific techniques integrated into archaeological studies add greatly to our understanding of complex geomorphologic processes and the response of indigenous societies to challenges posed by climate and landscape limitations. For example, 14C and luminescence dating (Berger et al., 2009; Watkins et al., 2011), as well as pollen and biometric analysis of sediment layers in canals and reservoirs, provides insight into age, use history of water control features (Huckleberry, 1999; Wright, 2003; Wright et al., 2005; Wright, 2006), and an understanding of crop types farmed by different societies. Additionally, much has been learned about prehistoric canals and fields through analysis of the physical-mechanical properties of sediments and alluvial deposits (hydraulic conductivity, porosity, stratigraphy). These studies provide insight into rain infiltration and seepage rates, as well as details illuminating the formation processes of canals and reservoirs. For example, sedimentological and stratigraphic analyses of Anasazi mesa top and valley water storage reservoir systems and canals were essential to understanding their role in sustaining local farming communities (Rohn, 1977; Wright, 2003; Wright et al., 2005; Wright, 2006). Application of concepts from fluvial geomorphology (Knighton, 1998) (e.g., erosion initiation, sediment transport and deposition) has proven useful in recognizing the impacts of floods and climate change on indigenous farming in the American Southwest (Cooke and Reeves, 1976; Bettess and White, 1983; Abrahams, 1987), as well as post-abandonment weathering of agricultural landscapes. When combined with dendrohydrological studies (Dean and Robinson, 1977), fluvial geomorphic analysis provides insight into how indigenous Southwestern societies changed their irrigation strategies (which are detectable from archaeological studies) as an adaptation to climate and landscape changes.
A survey of urban/agricultural water supply systems of major New and Old World societies on four regions of the world reveals exploitation of different varieties of water sources available in different ecological zones. Dams, reservoirs, canals, aqueducts, pipelines, open channels, and groundwater resources served to collect, transport, and distribute water to urban centers and agricultural fields. Each water system type with its selection of water transport and storage systems exhibited vulnerabilities when subject to climate and geophysical landscape changes. When system modifications were not possible due to insufficient technology, labor shortage, or lack of management expertise, societies underwent collapse, transformation, and altered societal and cultural trajectories as observed in the archaeological record. Differences exist between water transport and distribution systems employing different construction techniques and materials by New and Old World societies. Where the effects of climate and geophysical landscape change were minimal over long time periods, construction was permanent and alterations remedial in nature; where climate and weather patterns were changeable and affected the stability of water transport systems, flexibility of design and modification is evident to guarantee sustained use of these water systems. Examples discussed reveal this basic strategy difference between Old and New World societies. The many different water usage strategies employed by these societies constitute a virtual library of solutions tailored to different ecological and geomorphic conditions and provide insight into the creativity and resourcefulness of ancient engineers to maintain their communities despite changes in environmental conditions affecting their agricultural resource base.
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