Today's 'what's that?' moment

One of the bonuses of lockdown is being obliged to do the same walk time and again, and it's amazing what's new to see. Hanging over the garden wall of at the Manor in Hemingford Grey today I saw this device, which is presumably to regulate the channel between the river and the garden, which I've walked past, and visited the garden many, many times but never seen this. I guess it's an old flood defence. I'd love to know more if anyone knows about drainage.  

In extras I've added the colour version because I like it, and for context, the Manor reflected in the flooded garden. Said  channel is half way up on the right side of the edge of the bigger flood area, between the two puddles.