It’s a bit tricky due to all the splits in the river between river (proper) Lea and Lee Navigation.
Bear in mind KW itself is at a split between Nav & River. Above is the Nav, then the weir and split into river Lea and Nav:
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I think the nearest upstream gauge is Rye Bridge at Hoddesdon. Here you can see the gauge up top and KW down the bottom.
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That gauge has both level & streamflow (m3/s):
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As for fishing it, it can be difficult this time of year as it can be bollocking through. Bear in mind it’s difficult year round lol. Also depends which swim you’re in and where you’re fishing (in the flow, margin etc). If you’re in WB1 or EB1 an want to fish the sill, then yeah you’re gonna need a bit of lead.
I fished it Feb last year and it was in flood. WB1 was closed as it was washed away. You can see it underwater on left of this pic
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Make sure you use strong gear!