My guess is Marsh Wren. Your description, including behavior, is consistant
with this species and most probably a hatching year bird. These birds often
lack the lateral striping of the mantle and appear quite dark. I have a very
real hunch that most (if not all) House Wrens reported on CBCs are really
Marsh Wrens of this type. Your bird certainly sounds like a "stripeless"
Marsh Wren.

The behaviors you describe are not consistent with the Waterthrush group
(Though I believe one was picked up on an Oregon count this year).
Waterthrushes behave like wagtails and pipits (and Palm Warblers). Your
description is a very wren-like behavior.
>
>Description:
>Smaller than a Song Sparrow and larger than a Winter Wren. About 5".
>Dark brown above, particularly in wings and tail. On its back, a patch or
>patches of a lighter, redder brown...rufous or rusty, perhaps. Breast and
>belly probably whitish or pale brown, possibly with rufous tones in flanks.
>Unknown whether streaked or spotted. Buffy undertail coverts? Tail entirely
>dark brown (that is, no pale tips, no white edges) and proportionately
>short. There may have been a whitish eye line of some kind. Bill probably
>slender, slightly decurved, and possibly yellow or paler at the base.
>
>

--
********************************* In a world of sin and sorrow, there is alway
* Mike Patterson, Astoria, OR * something to be thankful for; as for me,
* mpatters at ednet1.osl.or.gov * I rejoice that I am not a Republican.
http://www.pacifier.com/~mpatters -H.L.Mencken