Dorset's diverse geography ensures it has an assortment of rivers, although a moderate annual rainfall coupled with rolling hills, means most are typically lowland in nature. Much of the county drains into three rivers, the Frome, Piddle and Stour which all flow to the sea in a south-easterly direction. The Frome and Piddle are chalk streams but the Stour, which rises in Wiltshire to the north, has its origins in clay soil. The River Avon, which flows mainly through Wiltshire and Hampshire, enters Dorset towards the end of its journey at Christchurch Harbour. The rivers Axe and Yeo, which principally drain the counties of Devon and Somerset respectively, have their sources in the north-west of the county. In the south-west, a number of small rivers run into the sea along the Dorset coastline; most notable of these are the Char, Brit, Bride and Wey.