I try to get out to the Black Hills at least once a year as I am about a day's drive from there. The last time I was there was spring of 06 and we did not notice Didymo as much as the glaring lack of water out there. We had good fishing in Castle creek, both above and below the Deerfield reservior, in Spearfish creek (which you need to see if you have never fished it), and in Rapid creek above Pactola (which is another beautiful place to fish). If you want to take the first exit off 90 in Wyoming, you will find Sand Creek which is an awesome fishery if it has water. We did not go to Rapid Creek below Pactola. Something started killing off the brown trout there about 3-4 years ago and I never heard if the DNR ever figured that out. That's why they are stocking bows below Pactola. Didymo is another strange thing that has naturalists puzzled, it is native to the Northern Hemisphere in North America and Europe. It has been imported to NZ where it is a very nasty invasive species and has ruined some fine trout water. If you haven't heard of it, Didymo is basically a slippery algae that will grow to cover everything. The odd thing about it is that even in it's historical native range, it sometimes goes wild, acts like an invasive species and just overwhelms everything else in the environment! If you find it in North America in some body of water there is no telling if that is just a normal occurance or if it is 'super Didymo' (unless of course it has obviously overwhelmed everything). There are a few theories about what is making Didymo go wild. It is probably not too great a stretch to say that the low water in the Black Hills has concentrated the nutrient load and allowed greater light penetration below Pactola which would contribute to the situation. Even though it may not be invasive in North America, you need to be really careful not to spread it around from places where there is an infestation. One theory is that it has mutated and the 'super Didymo' is some kind of mutated strain. The stuff is perfectly adapted to being spread around via felt soled wading shoes!