Although your first exposure to these streams will be in Portland's delightful fall drought period, subsequent monitoring and observations may take place under more trying conditions. Here are some safety tips:
Don't plan to sample from an area that is right by the stream because it will be very difficult to get to when the conditions are muddy and the stream flow is up.
Do not wade into the stream to get samples when the flow increases.
Unless you are sampling from a bridge or path, please have someone with you.
You should learn to recognize poison oak.
It is much safer to venture off trail or along some of the more remote sections of stream with a buddy. This can help with water related issues or unsavory characters.