Now that I have a full-size backhoe, I remove the muck down to the gravel base, including an area wider than the stream where it is soft on top, and lay the block on that base and a bit up the bank. The big risk is that the block most downstream dislodge during flooding, This is why I lay the block parallel with the flow of water (to resist tumbling), but sometime I need chunks of concrete downstream to stop that anyway. I also smooth out the tops of the blocks by pressing everything down with the back of the backhoe bucket, which cuts down on hand digging before laying them. I did not have the backhoe when I built my first one, but I was 30 years younger. You still get wheel tracks when everything is real wet, especially if sand deposits on the banks, but it's not too bad. I have had poorer results using culverts before I had the backhoe because water was able to work its way through outside of the pipes due to poor compaction (by hand), or passes over during flooding. The fords hold up better for me and reduce debris getting stirred up when crossing. Our main goal is wildlife habitat restoration, and this seems most compatible with that, These are really drainage ditches, but when all you have are ditches, you call them streams:) The price of block has almost doubled, as has diesel, so they are a lot more pricey to construct now. I have put most projects on hold, but wanted to get this one done while I can still move block.
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