Now, I hate to be a ninny, but with snowmelts fluctuating, and dams changing the flow and depth of rivers around Oregon, it's always best to test the water before leaping, even if you jumped there the summer before.
The first rule: Locals know best. Is there a tavern or convenience store nearby? Ask the clerk or barkeep. Is anyone else swimming in the area? The current may have carved out a hole in the riverbed, giving a better, deeper target. Ask!
The second rule: Pencil dives (straight legs, arms pinned to side) are discouraged. Pencil dives pierce the water's surface and send the jumper hard and fast to the river's bottom.
The third rule: Never dive headfirst on your first leap! What's an easy way to remember this rule? Think: Paralysis wipes out all feeling from the waist down. Consider your sex life: Is it worth the gamble?
That said, there are a few good bets around Portland. Even though the water in the Clackamas River never really warms up past the temperature of "bracing cold," the heart-stopping alarm of jumping 30 feet from a trestle and out of the hot summer air is worth the shock. From I-205, take the Estacada Exit; follow Route 224/212 until the roads split. Follow 212 to the right and down towards the river.