Relentless fun grade 3 with drops of 4 for the first ~1km especially in high water. Eases to grade 2/3 for the latter ~2km. It is possible to put in higher up if you want a warm up, you'll need to walk for it though.
From Mosedale bridge, head north a matter of metres, before taking a left up the small road towards Swineside. There is a reasonable parking area at the put-in near Grainsgill Beck, but be careful if your car doesn't have much ground clearance!
Rainchasers is best experienced as an app.