Hi, without a photo it is hard to determine which insect you have, but it does sound like fall webworm, Hyphantria cunea. There are other reports coming in for this native insect in Portland. Fortunately it isn't an invasive species and it is not generally considered a pest, as deciduous trees rebound from the defoliation caused by this insect. If the webs bother you, you can try using a hard spray of water to try to knock them out of the tree. Or prune off and dispose of affected branches. It is not advised to try burning the webs/affected branches. Whether you control the insects or not, your tree should produce a fresh crop of leaves next spring, unless there are other stresses present, such as too much/too little water, lawn mower/weed wacker damage to trunk, root damage from equipment, etc..
http://ento.psu.edu/extension/factsheets/fall-webworm
http://extension.oregonstate.edu/gardening/node/977

Wyatt Williams
Aug. 16, 2016, 6:16 a.m.

One more point, exotic gypsy moths, which are invasive species and not known to be established in our region, do not build webs like fall webworm.
https://www.oregon.gov/ODF/Documents/ForestBenefits/ODF%20Gypsy%20moth%20fact%20sheet%20Feb%202016.pdf

Wyatt Williams
Aug. 16, 2016, 6:19 a.m.