Dumb question about tidal currents

So if I’m paddling in a tidal river or narrow estuary and the tide is going out, there is a tidal current that draws me out to sea. Why isn’t one drawn further out to sea when in the ocean itself. The water level drops by the same amount (more-or-less) in both locations, but in one case I’m fighting a tidal current and in another case I’m not. Assuming for the moment that my premise is correct, I don’t logically get why there is a current in one case and not the other. My best guess is that my premise isn’t quite right and maybe one only gets noticeably pulled seaward with the outgoing tide when water from an incoming tide has accumulated in a large area and then gets flushed out through a narrow area (e.g., an inlet(/outlet) between barrier islands) - sort of like putting your thumb over the end of a garden hose to make the water come out faster. Is that it?

There must be at least some tidal current even at the beach though, right, because if the water level drops 6 feet, all that water that my kayak is resting on top of is going somewhere (out to sea).

yeah that’s it.

Same water dispersed thru a larger, less contained area.

Tides in most places are not channeled through narrow passages. SF bay, for example has tidal effects that intrude well over 30 miles into a massive volume of water. Put yourself outside the GG bridge and you’ll feel a tidal current well over 5-10 miles out to sea. On the opposite end of the spectrum, you have large rivers (Mississippi, Amazon, Nile, Columbia) where tides clash with a river current that greatly exceeds the rate of the incoming tide. Sure, the tide comes in, but there is a war at the interface that creates some pretty dramatic effects in the ocean.

As Celia pointed out, oceans have a vast area that allows the tide to return to sea, so the current is a smaller factor (it is still there). Thus tidal effects are more localized and don’t extend very far out. Throw a floating object into the ocean (the old message in a bottle) and if the tide is going out, it may stay out to sea (if it can get out far enough and can catch a different current before the tide, weather, and wave action change).

The other issue is that tides are not as deterministic in the ocean as they are in bays, inlets, and rivers. Ocean shores have longshore currents, tidal currents, local currents (tide rips), and often, lots of wind. All of these have an effect on floating objects that will probably exceed the effects of the tides. Tides here in SF Bay are pretty dramatic since any time you take the top 4-7 feet of water out of the bay (an area that is 60 miles long N/S and 30+ W/E) and pass that water through a 1 mile channel, you are going to get a rather appreciable current.

Try the Nakwakto Rapids if you want to see a tidal current that is channeled between islands in the open ocean. Look it up online.


Watching the current distribution in a draining bathtub provides an example …

Hey Dave, how’re you liking that Grand Illusion by now? Not to change the subject though; I’ve been enjoying the tides lately in the Columbia. The timing has been just right for catching a ride both ways if you timing is right.

We’ve been getting some rain lately and I haven’t been paddling since last Saturday, but at least the sky has been cleared of the smoke from those fires.

Back to tides: After dealing with tides on the Columbia for many decades, I’ve noticed a dramatic difference this year; the lows are extremely low as are the highs. This does not comport with the Antarctic melting off.

Hey Magooch, I’m loving the GI but haven’t yet had it out in the conditions in which it supposedly excels (the conditions which I tend to avoid). I’ve been in some moderate chop and it felt a little more secure than my Chatham. Eager to get down to Charleston where there can be some decent surf (not necessarily like you guys have on the Pacific coast) and significant claptosis where outgoing tidal flow is bouncing along some sandbars and deltas and meeting longshore current etc., to bring it back around to above discussion. Not sure when that will be though. As I’ve been away for nearly 4 months, I have a lot of catching up to do on less recreational activities! I did have some nice experiences on the way home, paddling at Pictured Rocks and the Apostle Island sea caves in MI. I couldn’t have asked for better weather. On the day I did the sea caves, the water was like glass which, as I understand it, is not all that common for Lake Superior especially as one gets into late Sept.

PS - after I’ve the GI for a few months, maybe a year, and have had in some different conditions, I hope to post a review.

Tides do cause depth to go up and down, and to do that, water needs to move in and out. So any place with tides will have currents. The more tidal change, the more chance of current.

But, the amount of current the tide creates will be more affected by constrictions and the amount of volume behind the constriction that needs to change.

Going to use my area of NorCal (SF area) as an example. We get maybe 6 feet of tide change, 2 high and 2 low tides per day. So over 6 hours, the water increase by 6 feet, then drops by 6 feet the next 6 hours.

Being out in the middle of the ocean is the non-constricted example. Yes, we gain 6 feet over 6 hours, but sitting in hundreds of feet of water depth, the motion can spread out and come from all over. So effectively you will have no noticeable currents. Any tidal current you may possibly fee would be covered up by other factors, such as the off shore currents (southerly in our area), winds, etc.

Now if we look at the SF Bay itself, we gets lots of constrictions. Here is a link to the current charts for the area:
https://www.dropbox.com/s/9v79g8mqzgys5s8/Tidal_Current_Charts_SF_Bay.pdf?dl=0. I would look at page 5 (max flood map) - you will see the fastest currents are right under the Golden Gate Bridge. That 1 mile wide gap is a significant constriction, and there is a lot of volume area behind it (inshore) that needs to gain 6 feet of water. So the water speeds up as it goes through the constriction. Go a little further in where thee bay gets wider, and the currents slow down. Go a little outside the gate where the ocean also gets wider (less constriction), and the currents slow. Fastest is at the constriction.

Others have mentioned about how you can see these currents from river mouths miles offshore, and that is more related to momentum. The current has a momentum as it goes out, and wants to continue straight until something forces it to stop.

@Peter-CA said:

Being out in the middle of the ocean is the non-constricted example. Yes, we gain 6 feet over 6 hours
The tidal range is much less than 6 feet in the middle of the oceans. The large tidal ranges that we see near coasts are caused by the tide hitting a continent where it can’t get any further.

Open ocean tide (which is really a long period wave) is only about 18" - 24" in open ocean. Oscillation within a body of water, when the tide reaches shore, like how water splashes high up the side of a bathtub, accounts for the large tidal ranges in some areas.

Thanks to @Allan Olesen and @gstamer for the clarification - I hadn’t really thought about tides and open ocean.