What does this mean? A creek channel is underwater lol so I don’t get how it’s flooded

That means it was a creek before the lake was created. The lake "flooded" the 

creek bed.

I thought that was all creek channels or is there a difference 

The contour or channel of the old creek remains after it's flooded. Think of it this way, you have a mud puddle with a flat even bottom and you take stick and drag it through creating a ditch or furrow you now have a added feature on the bottom of your once flat bottomed mud puddle.

22 hours ago, GoneFishingLTN said:

I thought that was all creek channels or is there a difference 

Think of this way. Any flowing body of water has a channel. From the smallest rill and brook to the Mississippi. Now erect a dam and create a reservoir. The original creek, river, etc. is now under feet of water flooded by the reservoir but the original channel is still there. And it is definitely structure. 

Kind of unrelated but looking at Google earth pro at a lake, reservoir etc., you can click on a time table tab that lets you choose different year satellite imagery that at times you can see what was before as a steam or what not. As you move forward you see how it filled up and those streams(channels) are still there just now flooded by the the lake. It’s very interesting and also helpful, for myself anyway .

Before & after...channel hasn’t moved...just flooded.

 

A picture is worth a thousand words.