Lava rock probably wouldn't be the best choice for waterfalls, because of all the reasons folks have already mentioned: it's too porous, too sharp, and too light. In addition, it tends to be fragile, so you could end up building something nice, only to have bits break off it and ruin your design.
Heavier, blockier rocks are really a better choice, but they don't need to be huge boulders. You can get quite a nice impact using a couple of larger rocks and then a scattering of smaller ones. For example, check out this build by the Aquascape team using basketball-sized rocks:
Also, if you can get limestone where you live, it's less dense (and thus lighter) than granite. Feather rock is even lighter. In my build, I've used limestone, as that's what came out of the hole as I was digging.
You can get more bang for your buck if you put two rocks that have similar textures next to each other and fill the space between them with crushed rock or similar. This will make it look like you've got one large rock with a fissure running through it. Another trick is to make a spillway with a flat stepping-stone type rock set vertically, with the liner brought up behind it and covered with flat rocks set horizontally. The vertical stone will look like the front face of a large square rock, while the horizontally set ones at the top of the falls will look like parts of the same stone poking up out of the stream.
I'll post an example below. Here, the spillway looks much more massive than it is, and so do the frame rocks. In reality, each of those rocks is set vertically, with other rocks set horizontally behind them.
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And here is what this looked like while being built:
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All three of these rocks were too heavy for me to lift, but I was able to roll them into place one at a time. The frame rock on the right was a two-person rock; I had my Larger Half manhandle it into the spot while I lifted it with a crowbar and added shims under it to hold it in place. After filling the voids with scrap rock, I added another fairly thin rock behind the spillway to make it look as if the spillway stone extended back a good ways.
Once the water was running and I could see how it was falling, I took a chisel and knocked off the the part of the spillway that sticks up on the right, so that I could make a divided falls by putting a rounder rock on top. One nice thing about using limestone is that it's fairly easy to carve with nothing more complicated than a hammer and chisel!
There are some tricks that can help you move the heavier rocks without throwing your back out. I'm a fairly small 61 year old granny, but have managed to move some pretty big rocks for my build. Some of the tools that have helped:
- Several heavy duty construction slings of different sizes.
- A strong wheelbarrow, sack truck, or other wheeled conveyance.
- A manual winch (come-along).
- A couple of good, sturdy crowbars. You'll end up using more than one at a time in most cases, often with bits of wood or stone as wedges and fulcrums.
- A snatchblock as a force multiplier. There are some great videos out there on how to use them. One important point is that the snatchblock is always attached to the object to be moved, *not* to the immovable support point.
- Some strong boards and pipes to use as rollers.
One word of advice is never rely on your muscles to do the work. Rather, use mechanical aids as much as possible, and be very careful and deliberate in your movements. What might have been a minor sprain when we were younger can easily put us out for months now (or worse). If you can finagle an assistant, that's even better!
