The longer of the sources rises in Rowney Wood, on the hill to the west of Debden Green, only a few hundred metres to the south east of the source of the River Cam that heads north through Cambridge eventually emptying into The Wash.[2] The River Chelmer flows past Thaxted, south through the district of Uttlesford around the northeast of Great Dunmow.

Work then commenced on constructing the navigation from Chelmsford to Colliers Reach in the tidal estuary of the river Blackwater.

According to Edward Arthur Fitch, the Fullbridge (the Chelmer river crossing at the bottom of Market Hill, Maldon) was a shallow ford.

For example, an island that is shown on the 1777 Chapman and Andre map of Essex[6] was moved from the middle of the river to fill in a curve to the north near where Tesco is today.

Further upstream, near the golf course, similar earthwork is visible and, from that point to Beeleigh weir the channel appears to be a cut bypassing the original course of the river.