1 is where the wall meets the footer
There is no 0, but you will sometimes hear some of the veteran boaters refer to levels such as minus 1 or minus 1/2. They are referring to an estimated level below 1 (at the wall/block meeting). The newer generation simply estimates the level between "fish flow" (minimum flow) and the first reading on the gauge (1) by saying things like "it's 1/2 foot". That 1/2 foot is essentially the same as the old reading of minus 1/2 (since the whole scale starts at 1, like medieval mathematics)
Everything else is pretty straight forward. There is an old worn out mark for 1 1/2, which is a great play level. 2 is clearly marked (another good play level, and probably the best level for first time Stairway runs since it's as padded as it can be without water going over Mushroom Rock).
This is a long-winded, round-about way to announce that I took a picture of the gauge this morning because someone had scrubbed the silt away and re-painted it. As silt builds up, and the gauge starts to disappear again, you can paddle up to it, splash it with water, and you should be able to see the numbers.