An arm length leader for Alton Baker? I've been from spillway at I-5 all the way to the spillway leading into the Willamette in a small boat, the only place you'd need such a long leader is about an 80 foot stretch just before the duck ponds start (near the canoe rental hut.)
I'd suggest no longer than an 18-24 inch leader anywhere in that place, especially if you're floating a bait on a slip sinker (like powerbait)
As for places to fish there, starting from the Willamette and working towards Autzen:
Bike bridge #1 near the Willamette: Clip one or two 1/8 ounce weights, I forget the name the clip on ones that don't move, above about 18-24 inches of leader with an octopus hook or treble. Salmon eggs or fluorescent power bait catch lots of trout after stocks here casting in the rapids under the bridge.
Islands: Cast between each island and a bit towards the Willamette here using a #2 octopus hook, no leader or weight, with a small ball of bread for carp.
Bike bridge #2 to car bridge: Standard bullet slip sinker over an 18 inch leader, eggs or fluorescent power bait = trout. Floating a couple salmon eggs under a bobber works here too.
All water between the car bridge and the canoe shack is prime carp territory on summer evenings. I use a #2 octopus hook with a small amount of bread, nothing else. Sight fishing here is fun too, the carp will even nibble at dry flies sometimes.
Canoe shack to the spillway only gets 5 feet deep or so, getting shalow as it moves towards the spillway then getting 3-4 feet deep again in front of the spillway. It's the same here as the second bike bridge, slip sinker above an 18-24 inch leader with eggs or fluorescent power bait.
The evening of the day after a stocking is epic trout catches with a white or black 1/8 ounce rooster tail in the entire canal. I counted once and lost track in the 50s XD