Annual rainfall ranges

Annual rainfall ranges EX527 from frontal Himalayan values of almost 200 cm to only ∼23 cm on the Indus plain, and even lower values (∼9 cm) over the Indus Delta. Tectonics control

the container valley geometry of the Indus, and the main course of the Indus migrated to a generally more westward located course over the past 5000 years (Kazmi, 1984). The legendary Saraswati River, whose probable ancient course in the Thar Desert is marked by numerous abandoned archeological sites, may have once supplemented the Indus Delta (Oldham, 1887, Oldham, 1893, Stein, 1942, Lal and Gupta, 1984, Mughal, 1997 and Giosan et al., 2012). Rather than being an effect of Saraswati’s loss, we speculate that a westward migration of the Indus course may have a more deep seated cause, possibly associated with slow flexural uplift of the central Indian plateau

(Bilham et al., 2003). The delta’s climate is arid sub-tropical; the river mouth is located almost in the tropics, at 24° N 67°30′ E. The present Indus Delta is 17,000 km2; the active tidal flat area is ∼10,000 km2. The delta once hosted the world’s largest arid mangrove forest (Inam et al., 2007). Warm coastal waters (22 °C on average) and summer tidal inundation result in salt deposits (Memon, 2005). The tidal range is 2.7 m (Giosan et al., 2006). Swampy areas on the delta are restricted to areas near tidal channels and coastal areas that undergo tidal flooding. Although the Indus B-Raf inhibitor clinical trial Selleckchem Gemcitabine Delta receives high deep-water wave energy, attenuation on the shallow shelf results in lower wave energy at the coast than is typical for wave-dominated deltas (Wells and Coleman, 1984). Wave measurements offshore Karachi at 20 m water-depth show a mean significant wave height during the summer southwest monsoon (May–September) of ∼1.8 m with a mean period of 9 s (Rizvi et al., 1988). During the winter, with offshore-directed monsoon winds (October–April), significant wave height

is ∼1.2 m with a period of 6.5 s (Rizvi et al., 1988). Wave-driven sediment transport redistributes river-delivered sediments along the deltaic coast (Wells and Coleman, 1984 and Giosan et al., 2006). Recorded regional history extends back several thousand years (including annals from the time of Alexander the Great c. 325 BC). Embracing ∼2 millennia prior, humans certainly modified the landscape: the population of the Harappan culture is estimated at ∼5 million at peak, with ∼1000 major settlements in what is now Pakistan. However, we postulate these modifications are relatively minor compared to changes from 1869 onwards when artificial levees and great modern irrigation systems became established, population grew from ∼25 million people to the present ∼188 million (UN, 2012), and the Indus ceased to transport large quantities of freshwater and sediment to the delta and the sea. We here describe natural processes occurring in the presence of humans, but not so greatly altered by them. The Indus floodplain (Fig. 1 and Fig.

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