The Ashburton River, 70 miles in length, is formed by the junction of its north and south branches just above the town of Ashburton. The parallel courses of the two branches are only a few miles apart on the Canterbury Plains and flow down the lower lying junction of the major fans of the Rakaia and Rangitata Rivers. The north branch of the Ashburton River drains the Winterslow, Old Man, and Moorhouse Ranges, 7,000 ft high, which are a block of foothill ranges between the Canterbury Plains and the Lake Heron depression lying parallel to the west side of the plains and running between the Rangitata and Rakaia Rivers. The south branch drains the southern end of the Arrowsmith Range, crosses the south end of the Lake Heron depression, and skirts the south end of the Winterslow Range reaching the plains at Mount Somers.
Of considerable importance to agriculture is the fact that these two branches maintain a local perched water table at a depth of only a few tens of feet along their courses, both separate and combined, across the Canterbury Plains, where otherwise the water table is several hundred feet below the surface of the plains. Ashburton was named after William Baring (Lord Ashburton), a prominent member of the Canterbury Association.
by Alan Copland Beck, M.SC., New Zealand Geological Survey, Christchurch.