Re: [Paddlewise] Reading the water

Bernerts wrote:
> I have question for all you seasoned yakkers ( and please don't laugh) :
>  How do you read the current? One night some friends and I were out on a
> tributary of the Columbia River. Dh told me the current was ebbing so to
> head up river first. When we got to our put in spot, it looked like it
> was flooding so we headed downriver first. Our return trip was much
> tougher ! Dh was right. He's a tugboat operator so I should've listened
> to him. So why did it look like it was heading the opposite direction.
> There was very little wind. Anyone else have this problem?

Melinda, you have opened a small can of worms.  Paddling on/near the 
Columbia system means the tide/current model you need to work with is a 
little more complex than most areas.

First, some basic tide/current modeling:  the tide is a _vertical_ change 
in the surface of the water which moves from the ocean into the river. 
When the surface of the water is rising, we call that a rising tide, and at 
its peak, the tide is at its highest (= "high tide" on the tide book). 
When the surface of the water is falling, that's a falling tide, and when 
the water reaches its lowest point, that would be low tide on the tide book.

But, in order for the water surface where you are standing to rise, water 
has to flow in from somewhere, and that means there will be a _horizontal_ 
flow of water, associated with the tide change.  A horizontal flow of water 
is a current.  When the current runs away from the ocean (up the Columbia 
from its mouth) towards Portland, for example, that would be a flood 
current.  So, as the tide is rising, at a given point in the Columbia, you 
would expect a flood current.  And, as the tide is falling, an ebb current. 
  That's the basic tide/current model.

But, the Columbia system has two complications which affect how that model 
works:

1. The Columbia is a river, with continual fresh water input in the lower 
reaches from the Willamette River and from the flow out of Bonneville dam, 
principally.  If the Columbia were a long, narrow embayment with _no_ fresh 
water input, tides and currents would be simple:  as the tide rises, the 
current would flood until high tide, at which point the current would drop 
off to zero ("high slack").  And, as the tide falls, the current would ebb 
toward the ocean, falling again to zero at low tide ("low slack").  Then 
the cycle would repeat again.  But, because of the continuing input of 
fresh water into this system, there is a downstream component of flow which 
influences the currents.

Basically, the fresh water input from upstream reduces the speed of the 
flood current, and enhances the speed of the ebb.  Consequently, peak ebb 
currents are stronger than peak flood currents.  It was that enhanced ebb 
you were fighting to get back to the put in.

The fresh water input also _delays_ the onset of the flood, and _extends_ 
the run of the ebb, so high slack occurs an hour or two _before_ high tide, 
and low slack occurs an hour or two _after_ low tide, in the lower reaches 
of the Columbia near me (Astoria).  Up near Portland, the effects are 
similar, but you may not see any flood current at all up there, because the 
effect of the tide is smaller (we get 10-foot high tides down here; yours 
run up to 5 or 6 feet, max.).

In a nutshell:  looking at the tide book for Portland, expect the ebb to 
begin _before_ high tide by a couple hours, and continue _after_ low tide 
for a couple hours.  (Seasonal variation in river flow changes these times 
-- in times of heavy fresh water input, there will be _no_ flood current 
where you are.  Ugh.)

2. The second complication is that side-sloughs and backwaters with 
restricted openings to the main stem Columbia can be a little out of synch 
with the main ebb current/flood current and high tide/low tide regime of 
the open River.  What happens, using a rising tide as an example, is that 
the water surface in a side channel with a small opening will not rise as 
fast as the water level in the River.  After a while, the water level in 
the River will reach its maximum level (near high slack), and the current 
in the River will begin to _ebb_.  But, the side channel, because flow into 
it is restricted, can lag behind, and its water level may be _below_ that 
of the River.  So what happens?  Well, as you know, water flows down hill, 
so water will continue to flow _into_ the side channel, showing as a 
_flood_ current, even though the main River is _ebbing_.

This is the second thing that nailed you and your buddies:  when you 
launched, the current in your _side channel_ was genuinely flooding (see! 
you aren't crazy!), but when you reached the main River, it was ebbing. 
Later, as the side channel "caught up" with the River, it began to ebb, 
also, and you had to fight ebb all the way back.  No wonder you were so tired!

In a nutshell, to avoid this happening again, pay attention to the currents 
(as predicted above) in the main stem River, and ignore the current in the 
side channel, if you are headed for the main stem River.

Hope this helps, and I hope I did not tell you how a watch works when all 
you wanted to know was what time it was!  (Borrowing one of Craig's metaphors.)

-- 
Dave Kruger
Astoria, OR
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