There are some serious fly fishers who will fish in March but I would say later in the spring it gets even better.

Many of the rivers that get salmon and steelhead runs do not open until the third week of May to protect the young salmon and steelhead that are migrating to sea.  Most lakes open during the third week of April or are open year-round.

Check out regulations here:   http://www.dfw.state.or.us/resources/licenses_regs/regulations.asp