They then flow towards the centre of the Rochford Basin, a circular feature which may have been caused by an asteroid impact in the Late Oligocene or Early Miocene periods.

The river channels are designated as "heavily modified" from their natural state by the Environment Agency, who measure the water quality.

The basin is approximately 3.7 to 4.3 miles (6.0 to 6.9 km) in diameter, and its base consists of London Clay covered by a layer of sands, gravels, and river brick-earths, dating from the Upper Pleistocene period.

[2] Four main streams drain from the Rayleigh Hills, and flow in a generally easterly direction until they reach the Rochford Basin.

The way in which they are all roughly parallel until they reach the basin, and then change direction, together with the presence of a small chalk inlier near Stambridge Mill, when the chalk layer throughout the area is between 360 and 520 feet (110 and 160 m) below sea level,[3] has led Stratford to suggest that the Rochford Basin may be the result of an impact by an asteroid in the Late Oligocene or Early Miocene periods.

[9] However, the Strategic Flood Risk Assessment prepared for Rochford District Council includes a map of the tributaries of the Roach, on which the third stream is called Noblesgreen Ditch, and parts of the fourth stream are called the Hawkwell Brook and River Roach.

It then passes north of the Grove Playing Fields, and past Rayleigh Sewage Works where its flow is augmented by the discharge from the reed-bed purification system.

It passes under the Shenfield to Southend Line, and then over a weir, which ensures that water fills Rochford Reservoir, built by the Great Eastern Railway in 1904.

It then flows in a north-easterly direction along the western boundary of Southend Airport to join the Noblesgreen Ditch at Rochford Golf Course.

[13] The Hawkwell Brook rises on the edge of Hockley Woods, to the north of the former HM Prison Bullwood Hall.

Now officially the Roach, it turns to the south east, passing under Rectory Road, and continuing to Rochford, running broadly parallel to Ironwell Lane.

It then crosses beneath the railway and Hall Road, running beside Bradley Way to reach its junction with Noblesgreen Ditch by Rochford Reservoir.

A six-year archaeological investigation ending in 2008 concluded that much of the vessel still lies beneath 21 feet (6.4 m) of mud near to the Paglesham hard.

[12] Shallow-draughted boats can use Havengore Creek to access the river, but the route needs careful preparation, as there is a lifting bridge carrying the road to Foulness Island, which is only opened for a limited period either side of high tide.

[12] At Stambridge, the modern river transforms from a fairly small channel to a wide tidal estuary.

[17][18] The tidal pound covered around 3 acres (1.2 ha) and was filled by the rising tide through a set of gates similar to those on a lock.

The streams supplied some 950,000 imperial gallons (4,300 m3) of water per day to the pound, based on a catchment area of 7,650 acres (3,100 ha) and an average rainfall of 20 inches (510 mm) per year.

The solicitors acting for Rankins stated, however, that there was considerable leakage through the mill head, and the unshrouded wheel made their case much weaker.

The wheel suffered from subsidence in 1951, which prevented further use, and the mill burnt down in 1964,[20][19] two years after Rankins sold the operation to Associated British Foods.

Those creeks and tidal flows also enhance the River Roach's habitat and conditions for oyster cultivation and fish spawning.

However, like most rivers in the UK, the chemical status changed from good back to fail in 2019, due to the presence of polybrominated diphenyl ethers (PBDE), perfluorooctane sulphonate (PFOS) and mercury compounds, none of which had previously been included in the assessment.

[30] The Food Standards Agency imposed a ban on the catching of shellfish in March 2015, when extremely high levels of the pathogen E. coli were found in the reach at Barton Hall.

The problem was traced to faulty plumbing at a number of homes, which resulted in raw sewage entering the river.

Although Anglian Water were not responsible for the faulty workmanship, they rectified the problem, and carried out other work to the sewage system.

[31] Download coordinates as: Light aircraft based at Southend Airport use the Roach as a navigation aid to guide them back to the runway.

[citation needed] When flying visually the visibility is not always good enough to see long distances, so pilots use geographic features to confirm their position with what is shown on their charts.

The Roach flows past Purdeys Industrial Estate, Rochford, which is very close to the threshold of Runway 24.