The Shrinking River Dee
The River Dee was once much wider to the west of Chester, and quite large ships were able to sail right up to the city walls in Roman times.
Over the centuries, however, the Dee Estuary filled up with mud and silt, and larger ships could no longer reach the city. This made Neston and Parkgate into successful small ports, with many ships sailing from there to Ireland.
In 1732, Dutch engineers were given the job of building a new channel for the River Dee by local merchants and Chester Corporation. This was to make it easier for ships to reach Chester, and to reduce the silting up of the river. The straight section of the river channel they built, which leads to the mouth, is easy to see on the maps below - it is 5 miles long, and was called The New Channel when it was opened.
However, the silting up continued, and the newer port of Liverpool soon became much more important than Chester, because much larger ships could sail up the River Mersey.
Move your mouse over the map below to see
where the River Dee used to flow!
A map from 1777 showing The New Channel (straight section)
of the River Dee.
The old, winding course of the Dee can be seen above The New Channel - the green areas are marshland at the edge of the estuary.

The marshland was reclaimed from the sea, and today includes the areas of Chester known as Shotton and Sealand.

A map of Chester and the Dee from 1610
by John Speed
Look at  how wide the river was.
A recreation of Roman Chester
Massive stone blocks in the Roman harbour wall can still
be seen at the rear of Chester Racecourse.

Can you see the Roman sailing ships approaching the harbour on the left?
A view of The Old Port in 1855
by John McGahey
The picture shows many sailing ships along The New Channel, and around the wharves and warehouses which were built after the Dee was canalised in 1732-36.

Old Port View from Chester: A Virtual Stroll Around the Walls 
Chester/Cheshire maps from Cheshire Local History Association
Other maps from Google Maps
Model of Roman Chester from Sarah Woodbury: novelist