Modern Processes
Though precipitation and temperature have varied throughout antiquity (cf. Bar-Matthews, et al., 1998; Courty, et al., 2001; and Wilkinson, 2003), annual precipitation currently averages between 300 and 400 mm annually. During the late 1990s and early 2000s droughts decreased annual amounts to around 200 mm. As a result, the flow of Wadis al-Mujib and al-Hasa are reduced to little more than a trickle during the summer, with the Wadi al-Karak being completely dry.
A 1989 model developed by Montgomery and Dietrich proposed that basins with a low relief require a large source area for channel initiation. Their work also found that all else being equal, drier regions tend to have larger source areas. Their findings are consistent with what we find on the plateau.
The total source (i.e., catchment/basin) area draining into the three major wadis is 9,306 km2 (al-Mujib 6,596 km2, al-Karak 190 km2, and al-Hasa 2,520 km2) Wadis al-Mujib and al-Hasa together draw from over 87% of the total basin area east of the Dead Sea with their reach extending nearly 120 km across the highland and plateau (MacDonald, 1973 and Natural Resources Authority of Jordan, 1987 in Salameh, 1997). The basin is drained through a combination of runoff flowing directly into the canyons, an extensive network of ephemeral channels, and sapping.
Elevation levels in meters along the escarpment and channels
Runoff Processes
Odeh and Salameh (1996) suggest "Much of the hydrology and geomorphology of the study area can be attributed to the thunderstorms of the upper watershed hill slopes and the plateau to the east which often leads to sudden and high magnitude flash floods, accelerating stream work. This is consistent with the results found in other areas by Bandara (1974), Smith (1994), Thornes (1994), Chorley (1957), and Rinaldo (1995).
Even though the overall tilt of the plateau is away from the general direction of the drainage flow (i.e., away from the Dead Sea), the overall topography is such that it directs runoff into the channel heads of the major wadis. This is especially true for the southward reaching channel head of the Wadi al-Mujib.
Channels extending from Wadi al-Mujib
Ephemeral Processes
In addition to precipitation draining directly into the major wadis, there is also an extensive network of ephemeral channels that feed the major wadis during pluvial events. Ephemeral channels are particularly well developed near the head of Wadi al-Mujib.
Sapping Processes
Though the majority of precipitation, both past and present, has drained directly into the major wadis through runoff processes, sapping appears to be the dominant shaping force at work along the rim of the plateau (See Luo, et. al., (1997) for a discussion of sapping processes along the western desert of Egypt, and cf. Oberlander’s (1965) discussion on sapping in the Zagros Highlands). Sapping is defined “as the process leading to the undermining and collapse of valley head and side walls by weakening or removal of basal support as a result of enhanced weathering and erosion by concentrated fluid flow (i.e., ground-water) at the site of seepage.” (Laity and Malin, 1985). Evidence for sapping processes on the plateau can be found in the numerous theater-headed valleys that are found all along the sides of the major wadis. Given the high incidence of fractures along the many faults of the plateau, it is suggested that the source of the seepage is the subsurface flow of water along fractures in the bedrock (cf. Anderson, et. al., 1997). This conclusion is supported by the findings of MacDonald and the Natural Resources Authority of Jordan. Their research (reported by Salameh, 1997) found that fractures in the sandstone aquifers are the principle conduits for the transport of subsurface flow across the plateau.
Channel development on the Wadi al-Mujib
Conclusion
The Karak Plateau provides an ideal case-study for observing hydrological processes and their impact on the development and preservation of drainage networks on uplifted features. The plateau exhibits both ancient and modern landforms associated with drainage systems and their morphology. The courses of the Plateau’s channels were defined millions of years ago during periods of increased geological and tectonic activity. The magnitude and expanse of its channel canyons is the product of oceanic and pluvial events driven by fluctuations in climate beginning 100 million years ago and continuing today. The Karak Plateau stands today as witness to the ancient past that carved and shaped it; a witness that is preserved and sustained today by processes common to nearly all physical environments (i.e., overland flow and sapping).