If water trickles down a small waterfall and there is gravel behind the rock wall down which the water falls over - will good bacteria be able to work with water flowing directly through the gravel?

Any wet gravel or rocks will grow the bacteria on them. The rate of water flow doesn't matter unless it's so fast as to scour everything off the rocks which is not likely for most ponds. Rocks through which doesn't flow directly but are still damp from splash action will even grow some bacteria. Rocks like that in my pond grow a lot of moss which helps filter the water too.

I tested out my 600 gph pump today, and feeding water through a garden hose for a tested distance of 14 feet at 20 inches high (simulating the furthers distance my water fall will be from the pump and the must hight it will have) - it created a descent flow which would definitely cover my water fall at a descent pace - however, if i make my bog 4 x 4, 5" deep (one inch water over 4" gravel) the pace of the current over the bog gravel will be very slow and disspensed in a diluted way - but i dont think any of the 4 x 4 feet will stagnate - so should i keep it at 4 x 4 - i dont care about the flow, as long as the biofilter is successful. Do you think a water fall of 20" trikling in a 5" spread over a steep, nearly straight field rock pile verticly would create enough oxygen for them. to tell you the truth, if i didnt have a waterfall and a long distance to send the water through a hose and only left the pump in the pond the water current would be too strong for my perhapse only 400 gallon pond with a 600 gph pump.

Since the flow rate doesnt matter, i guess you are just saying that biofilters need OXYGEN, not necessarily flow; only that flow tends to have the most oxygen, right? In that case, if this is what you are saying, will gravel in the deepest part of my pond be a substrate for some bio filtration if say the plants produce a lot of oxygen?

600 gph is not too much movement for a 400 gallon pond. My 153 gallon pond has a 700 gph pump!

Nitrifying bacteria need oxygen which can be obtained by water that is previously high in oxygen moving over it or has its oxygen increased via water movement (splashing, waves from movement, etc.).

A separate issue is the water flow that is needed to prevent stagnation and to allow water from the pond to have time with the bacteria to exchange ammonia to nitrite and nitrite to nitrate. The more water per time that flows past the bacteria, the more bacteria that can grow and the more biological filtration power that you have. That's at least until the flow is so fast that the bacteria either don't have time to do their thing or are scoured off by the force. That's not going to happen in your case of course.

You can make your bog whatever size you want. You won't know how things work out until it's all set up and established.

Any surface in the pond - liner, gravel, plant pots, plants themselves, rocks, etc. will grow some good bacteria on them along with microbes, algae, etc. The areas with more oxygen and more flow (hence food source) will grow more. In the deep parts of the pond, unless you have an air stone down there, the oxygen levels will probably be low. Thus, anaerobic bacteria are more likely to grow. As biproducts, they produce methane and hydrogen sulfide gases which create pond stench. For a lengthy discussion on the various kinds of bacteria, stagnation, etc. that I had with another ponder, see http://www.fishpondinfo.com/talk.htm

Submerged pond plants will put oxygen into the water during the day but use it during the night. The other pond plants have gas exchange with the air and therefore, will not have an effect on the oxygen levels via photosynthesis (but plant substrates may effect the various sorts of bacterial growth and gas production/usage).

Water Flow - Pipe / Line sizes

Your 40 ft garden hose offers a lot of resistance to the water flow, and probably as much as the 20in head (height). I try to maximize flow at end of the line/pipe then deliberately restrict (valve/tap/plug) if its too strong. Most garden hoses are only 3/8in inside diameter, ocaasionally you can find a 1/2in garden hose.

Pipe/hose resistance is directly related to inside diameter squared of line - ie 3/4in pipe has 120% more capacity than 1/2in hose, or 45% of the resistance (plus/minus). Flow is also directly related (but not squared) to length of line. So to reduce resistance to flow in long lines increase line diameter.

I use 1in plastic pipe for 800 gph pump - the pump doesn't even notice the pipe is there (30ft). Actually I use 1 in. black poly tubing for everything, saves keeping different fittings on hand, easy to get standard connectors for garden hoses.

Lower the resistance -> increase height, increase length of line possible, increase the flow.

Robyn,

I'm using a 20 foot rubber hose for ponds and it is 1" diameter - it doesnt seem to effect the flow much at all - i can still hold it up way over my head and get great flow from my 600 gph pumpfilter. can you tell me what is the best way to maximize biofiltration- is it keeping the lava rocks in the waterfall at the top where the water is just fed into the water fall, is it keeping it spread out through the water fall, how about spread out throughout the water fall under the flow of current or set up as a wall of lava rock through which current passes, alternatively. or is it keeping the lava rock after the water fall course directly in the bog under 1" water

Any location where the lava rock is and stays wet, there will be some filtration. The more water that passes through the rock, the better it will be. So, I would put it in whichever of the areas that you mention where the most flow will go through (not just over) the lava rock. There's no reason why you can't put some in multiple locations either.

would it be bad if i made a pool at the top of the fall and filled it with 12" gravel and placed the hose right under the 12" of gravel and lava rock - would this not be a good idea because the water comming from the hose directly has not yet accumulated much additional oxygen since its comming straight from the pond water- if you say that i am right in concluding this is not the best solution for this particular reason, then i guess i would make permeable walls of lava rock at the mouth of each waterfall scaffold so that the water passes through the lave rock.

It is perfectly fine to put the initally piped in water directly into the bottom of a bed of biofiltration material (in your case pea gravel and/or lava rock). That is how most upflow filters work. Mine goes into bioballs and then lava rock before falling out into the waterfall. The waterfall oxygenates the pond so that water being pumped should still have enough oxygen to sustain a good amount of bacteria. Bacteria will also grow down the falls and stream areas as well. Just be sure that you can clean the area where the intital piping goes in in case it gets clogged. Those rocks would need cleaning every month or two since they will also be doing some mechanical filtration as well (depending on how good a pre-filter you have on your pump or system).