Most of our rivers in Spain are basically navigable from spring to summer when the river flows are at their maximum. Some rivers in the north are also navigable in autumn and winter thanks to the abundant rainfall they receive at this time of year, and some others that are starting flowing up due the melting snow.
We are coming from class I-II like Ebro river on the Catalan part, or Sella and Deva in Asturias.. to class III-IV in Spring time on the Noguera Pallaresa.
So, we could say that each one has its particularity, and in some locations if weather conditions are ok, is possible to have all year round activity.