Yes I got to agree with your explanation of near or at the surface and hanging between the pontoons. and I really like the Armor Weave but made from steel ! I've never used the heavy aluminum Armor Weave material and if your mainly doing the small aggregate material , I could see using it for the weight loss of using aluminum . BUT getting back to under water , if your trying to take advantage of the effects of not having to pump the water from deep depths and have the sluice close to the bottom, then a open top would not allow the water/material act / flow correctly in the sluice with riffles. The flow of slurry would be disturbed by the water flow from the stream and not let your riffles work properly. That's were I'm coming from with my post. I was once wanting to build one that screened of the bigger material (just like they do) and make a outlet for the fines to go to a fine screened subbie to help save them. Subbies have been know NOT to save fines, I wanted to change that thought BUT never got to it and had to give it up! I hope this will clear up where I was trying to go with this thought .