Y-axis units are the height of water in cms just upstream of the bridge at the put in.
From
Daithi's website, with typical understatement:
"70cm is just runnable, tough on gear, no big holes on the drops but
falling over in the wrong place could well require a trip to the hospital
100cm is medium, holes have appeared, swims are nasty
130cm and above is high, not for the faint hearted"
...or if you prefer, it runs from zero to scary.