I paddled the Usk last weekend in high water. It was over the banks, so
I'd reckon you can officially call it 'in flood'.
http://www.bristolcanoeclub.org.uk/p...005/index.html

But we were trying to decide what 'in spate' actually means, or is
assumed to mean. Any suggestions?

http://dictionary.reference.com/search?q=spate
This says that 'spate' means:

1. A sudden flood, rush, or outpouring: “It issues a spate of words
from the loudspeakers and the politicians” (Virginia Woolf).
2. Chiefly British.
1. A flash flood.
2. A freshet resulting from a downpour of rain or melting of snow.
3. A sudden heavy fall of rain.

--
Conor O'Neill, at home in Bristol, UK