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

I always considered spate as a lower water level than flood, spate is
bank full or just above, as much water as the river can carry without
actually flooding and withough bursting its banks.

Lots of rivers in The Scottish Guidebook are classed as spate runs -
you want them full but not ncessarily in flood.