Not All Is As It Appears

No this is not a canal, it is the road running alongside Swanpool Lake leading to Swanpool Beach. When I saw it I started to think about how the salinity will have been affected.
Swanpool Lake used to be part of the sea The area was cut off by a shingle bar which formed after the last Ice Age, and a freshwater lake was created. The lake was three times larger and it's water level three metres higher than it is today. The road that you cannot see under the water runs between the lake and the beach and sits upon the original shingle bar.
In 1826 a culvert was dug through the bar draining much of the lake into the sea and leaving the water level as it is today. Water still drains out of Swanpool through the culvert at the southern end, however, at high spring tides sea water rises above the culvert and flows back into Swanpool. As a result the lake is now brackish - a mixture of salty and fresh water.