Globe and Mail: The seasonal waxing and waning of rivers is one of nature’s most crucial cycles, influencing everything from the success of salmon runs to having enough water during parched summers to irrigate crops. But by this measure, many of Canada’s major rivers are in trouble, contends a new report that says many of the best known rivers have suffered major alterations in their natural flows due to hydro dams, irrigation schemes and withdrawals by industry, and could be further compromised by …