the welland passes to the north of corby near rockingham, and then to the south of caldecott, where it becomes the county border between northamptonshire and rutland, and the eye brook, which has been dammed to form the eyebrook reservoir, joins from the north. as it flows past harringworth, the river forms two channels, with the county border following the smaller, northern channel. it is crossed by the welland viaduct, with its 82 brick arches, which was completed in 1879, and carries the oakham to kettering line over the valley. apart from viaducts carrying suburban lines into london, it is the longest railway viaduct to be built in britain. uppingham brook flows eastwards from uppingham to join on the north bank, and the jurassic way long distance footpath crosses the river at turtle bridge. this probably dates from the fourteenth century, although it was widened in 1793, and a parapet has been added subsequently. on the road from barrowden to wakerley, there is a medieval bridge with five pointed arches, which was widened in the eighteenth century. gretton brook flows from the south to join the river near duddington, where there is a well-known mill building of earliest known date 1664. a limestone ashlar bridge with four arches crosses the river, dating from the fifteenth century but widened in 1919. after the river passes under a railway bridge at ketton, its flow is swelled by the river chater. the county border again leaves the river to the west of stamford while below the town the river forms the border between lincolnshire and cambridgeshire.