SCA Advice on Access to Rivers
This page used to contain detailed advice from the SCA on enjoying Scotland's rivers. However, the information was largely superceded by changes in the Scottish access laws, achieved in no small part through the work of the SCA.
Hence, the best advice is to read their most up to date info here on their website.
Also see www.outdooraccess-scotland.com and http://www.opsi.gov.uk/legislation/scotland/acts2003/20030002.htm