I was standing in the middle of a small river in the rain. The shot was great, but I knew the rain hitting the Kata cover over the AT4073 was [ ] the audio pretty badly. I adjusted the volume down to keep the spikes out of the red, and it still picked up the water and rain OK, but the drops on the cover are, well, distracting to say the least.

I wondered if I had a limiter in the loop, would I be able to set the on-camera (V1U) pre-amps high enough to pick up the flowing water while the limiter would prevent the spikes caused by raindrops.....well if I had a limiter I'd probably also have a pre-amp in the same unit, so then I could set the camera's noisy circuitry ALAP. And, in a rain storm, there are no quiet moments, so the 'camera's noisy circuitry' doesn't really play a role. But if I'd seen some interesting critter moving through the woods after the rain, when everything was still dripping, then the 'noisy circuitry' would be noticable, and the drops all the more so. I did not try using the AGC at the time.

Over at JuiceLink, the engineer did not add a limiter into his quiet circuit because the camera's AGC would take care of that function.

I always thought the AGC was made to boost quiet signals up, not limit strong signals down.

So this post seeks a deeper understanding of AGC...does it limit as well as boost?

Would a device with good pre-amp and limiter allow a recording of flowing water and rain on the water, while suppressing the overdriving sound of rain on the cover?

In a word "no." The only way to get clean sound in your situation is to have the mic off camera (well away from your rain cover) and covered with a serious mic muff or softie.

AGC is a relatively crude control for evening out sound levels - both soft and loud. Move the mic away from the offending rain cover noise and hire a sound guy with a mixer if possible.

There are some tricks you can do in post with sonograms that can eliminate or at least diminuate (is that a word?) ... ok .. lessen ... the rain noise.
But other than that, a limiter will raise not only the background sound, but of course also the transients from the raindrops.
What I would do in that situation, is to return to location if possible, on a dry day and record the background soundtrack audio only, and then dub it over the existing footage, and mix in some foley from rain, at least then you get to control the level.
Is that cheating? you bet it is :D but sometimes that is what it takes...

Just my 0,02 eurocents...

Depending on how long you are going to stay out in the rain, a thick layer of felt arched so that it doesn't touch the top of the camera would do for a little while, until it got saturated. Have fresh ones standing by. It's low-tech, but cheap!

Just put a big umbrella over your camera... hold it about 5 or 6 feet over your camera with a long pole or something (duct tape it to a broom handle) and it should be high enough that the sound of the rain hitting the umbrella would mesh in with the sound of the river you're in...