Tidal affects not previously mentioned by FlyJack: (1) reverse flows stagnant, weaker, and less velocity than normal flow rates, (2) dead zones of no flow, (3) post tidal ebb stage: normal current surges as released, normal flow rates increase and are stronger for a period as reverse tidal effects subside! (4) reverse flow propagates as both longitudinal and transverse waves (perpendicular to longitudinal wave flow) creating turbulence, cavitation, and other obstructive forces - particles suspended in a reverse flow are know to circle in eddies and go nowhere ...
TIDAL EFFECT ON THE COLUMBIA & WILLAMETTE RIVERS
In each 24-hour period the Oregon and Washington coasts experience two high tides and two low tides. These tides affect the Columbia upriver as far as Camas, but not much beyond, and upriver on the Willamette to Oregon City. Mean ranges of tides in the Columbia range from 6.7 feet at Astoria, to 1.3 feet at Vancouver and 1.8 feet at Portland.
Traveling upstream from the mouth of the Columbia, the effect of the tide takes approximately six hours to reach the mouth of the Willamette River. As this tidal effect works its way upstream, it slows or sometimes reverses the downriver current of the Columbia. When the tide changes to the ebb stage at Astoria, the holding action of the high tide against the river current is released and the river current then accelerates to discharge water held back by the tide. The same affect occurs to varying extent at all locations affected by tidal action
Tidal effects should be taken into consideration when anchoring, particularly in the lower Columbia where tidal rise and fall is more noticeable. To save time and fuel, boaters can use the forces of the tide to their advantage by timing the departure of a down river trip with the start of the ebb stage, and an upriver trip with the tidal release stage when unobstructed current resumes and is swifter.
Inexpensive tide books are available at most marine and fishing supply stores. These books list the predicted times of high and low tides for the calendar year. The books also give the times of high and low tides at various locations along the Columbia.
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