Drainage of the central Yilgarn System is confined within two north-south watersheds, the previously described Central Watershed and the Median Watershed defined here. Runoff today is either intermittent or inactive, but the entire system is gathered to a single outlet through the Median Watershed where it escapes to the Avon. The shape of the catchment with prevalence of north-south alignments, many directed to south coast rivers, suggests that the System originated with southerly drainage. The System is bounded on the south by a watershed parallel to the south coast which formed in the middle to late Cretaceous by marginal uplift of the craton during separation from Antarctica, causing reversal of drainage. There appears to have been a simultaneous down-warp along the line of the present Yilgarn River, the "Chin-Smith lineament". Before uplift the rivers incised canyons into the rifted margin of the continental platform. Subsequently the margin sagged towards the rift forming the Ravensthorpe Ramp and carrying down the valleys as submarine canyons. Along the western part of the south coast there is a secondary axis of uplift, the Stirling Range Axis, 50 km further south. On the eastern side there is evidence for a palaeoriver, the Woolgangie, which has been truncated by the Lefroy-Cowan System. Drainages to the west coast formed in the Mesozoic bounded inland by the Median Watershed. In the early Cretaceous, uplift was associated with continental separation. An extensive canyon was incised at the edge of the rift and later drowned. High sea level prevailed in the Perth Basin to the Late Cretaceous. Regression then promoted rejuvenation of the rivers. The Brockman, Mortlock North Branch and Avon owe their alignments to the Chittering and Jimperding Metamorphic Belts. In the Late Eocene, marginal uplift of the craton by 150-200 m in a belt 80 km wide brought up the Darling Range and caused diversions of drainage. Headwaters of the two Moore Rivers (north and east branches) were diverted to the south, as was the upper Mortlock (north branch). Further down, the Mortlock was relocated some distance east. The combined Avon and Yilgarn Rivers were diverted north, from a westerly course, to join the Mortlock. The Arthur and Beaufort Rivers were cut off from the Collie catchment and diverted to join the Blackwood. Depth of the palaeochannels here and beneath the Avon-Yilgarn afford proof of Darling Range uplift. Monadnocks in the Darling Range which have also been cited as evidence for uplift have been identified and mapped.