This is the estuary of the River Colne, in Essex.

Given the size of the estuary you are perhaps thinking that the river itself is a sizeable torrent but…

…this is the Colne, just about a mile from where it becomes tidal.  So how come the tidal part of the Colne leaves such an impression on the landscape?

The answer is that the Colne is a ria.  A ria is a river valley that has been permanently flooded by the sea.  The sea level has risen allowing it to come inland along the river and spill over onto the flat valley floor on either side.  And it is not just the main valley that is flooded but also the lower tributaries too.

I left the main channel to follow Brightlingsea Creek and then turned off again to follow the smaller St Osyth Creek.

Both the tributary and its tributary were tidal and much, much wider than you would expect from their feeding rivers.

So the rivers formed the valleys and then the sea rose and flooded them.  Thus the sea was lower in the past and, as we saw last week, this is evidence for previously lower temperatures, with water locked up in ice rather than in the sea.

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