Ultimately, Basra's irrigation canals were unsustainable, because they were built at too little of a slope for the water flow to carry salt deposits away. This required the clearing of salty topsoil by the Zanj slaves in order to keep the fields from becoming too saline to grow crops. After Basra was sacked in by Zanj rebels in the late 800s and then by the Qarmatians in the early 900s, there was no financial incentive to invest in restoring the irrigation system, and the infrastructure was almost completely abandoned. Finally, in the late 900s, the city of Basra was entirely relocated, with the old site being abandoned and a new one developing on the banks of the Shatt al-Arab, where it has remained ever since.