The River Chelmer flows entirely through the county of Essex, England; it runs 40 miles (64 km) from the north-west of the county through Chelmsford to the River Blackwater, near Maldon.

The River Blackwater is a river in Essex, England. It rises as the River Pant in the northwest of the county, just east of Saffron Walden, and flows in a generally southeast direction to Bocking, near Braintree, via Great Sampford and Great Bardfield. At Bocking, it becomes the River Blackwater, and veers east to flow past Bradwell Juxta Coggeshall and Coggeshall. It then veers south, flowing past Kelvedon and Witham, before reaching Maldon. There, it veers east again and empties into the Blackwater Estuary, which in turn meets the North Sea at Mersea Island.