Roadways: Highway 101 (Astoria-Megler Bridge)

Astoria-Megler Bridge, Highway 101, Astoria, OR
Canon Power Shot SX40 HS, 1/640s, f/4, ISO: 160

I love going across this bridge.  It spans the Columbia River near its mouth.  The river is massive at this point, roughly four miles wide where the bridge crosses it.  Living in the Bay Area I'm used to bridges of this size and length, but this bridge isn't going across a bay, it's crossing a four mile wide river.  How can a river be that big?