High Quality Content by WIKIPEDIA articles! The River Trym is a short river, some 4.5 miles (7.2 km) in length, which rises in Filton, South Gloucestershire, England. The upper reaches are culverted, some underground, through mostly urban landscapes, but once it emerges into the open it flows through a nature reserve and city parks before joining the tidal River Avon at Sea Mills. 18th-century water mills near the mouth gave the area its name.