Stream-side plant

I found these plants next to a stream in NorthWest Washington state.
The pictures were taken in Spring, and the plants were getting to somewhere around 6-7 feet tall at the time.

The shorter shoots were all coiled up.

Any idea what this could be?

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Eeeck!
It looks like the cursed Japanese knotweed. Fallopia japonica.
It was introduced to Britain by the Victorians, cos it had pretty flowers. Now it is everywhere! It will even grow up through tarmac and concrete.It is a very invasive pest.
Quote from wiki....." It is listed by the World Conservation Union as one of the world's 100 worst invasive species.[3]"
However, in Japan it is not a problem as there are natural predators there.

japanese knotweed - Google Search

http://news.bbc.co.uk/1/hi/magazine/4267426.stm

http://en.wikipedia.org/wiki/Japanese_knotweed