River delta slope offshore, waves will make river deltas smoother. Waves can also be responsible for carrying sediments away from the river delta, causing the delta to retreat. For deltas that form further upriver in an estuary, there are complex yet quantifiable linkages between winds, tides, river discharge, and delta water levels. Erosion is also an important control in tide-dominated deltas, such as the Ganges Delta, which may be mainly submarine, with prominent sandbars and ridges. This tends to produce a "dendritic" structure. Tidal deltas behave differently from a river- and wave-dominated deltas, which tend to have a few main distributaries. Once
Indus River Delta (however, the delta has lost 170,000 hectares of mangroves over the past 50 years). The delta faces pollution both from sea and the Indus river. Chemical run-off into the river threatens many species. Most of this chemical run-off consists of agriculture pesticide and fertilizer. The delta faces pollution from the Arabian Sea. Karachi, Pakistan's largest city, releases sewage and discharge from industrial units into the Arabian sea, most of which is untreated. Both Port of Karachi and Port Qasim handle significant shipping traffic, resulting in oil discharge, some of which reaches the delta. All of this pollution lowers the river