Here's some information based on my experience in filling a pool with river water, brook water, etc. YMMV of course, every water source will be different.
I filled my 30,000 gallon pool with 1/3 river water, 1/10th well water, and the rest (effectively 2/3rds) from a running stream behind my house. Chemically speaking, the well water was very high in Calcium... I knew that going in and that's why I limited the amount of well water. The brook water still showed 200ppm of Calcium the one time I tested it, which is still high, but manageable. Other than Calcium, everything else was manageable.... but, the dirt...
I certainly saved money by not hiring a water truck (~$2000)... but if this were swimming season I would have had some upset swimmers while waiting for the water to clear... it's been two weeks now and it's just starting to get to the point of seeing the bottom of the deep end (about 8'6" deep). There was a lot of very fine dirt/silt in the river water... I am using a small sand filter temporarily until I install a big one next spring, using a Hayward S210T with a 3HP Pentair VS pump... yeah, the pump can definitely blow everything through this filter, but I've used slow speeds only while filtering. (was kinda funny when priming the pump and it went full speed... the water from the return jet in the picture below was going all the way across the pool and hitting the opposite wall, splashing into the air!)
So... pictures are worth 1000 words... here is the water after filling, and again after about two weeks of vacuuming to waste, 24h filtering, lots of backwashing... and, I'll add some of my experience with adding DE to the sand filter... it made an amazing difference! I have a valve and water tap on my return fittings, so I filled a glass with water before adding DE, and immediately after adding DE to the skimmer, it made an immediate and very noticeable difference in the water clarity being returned to the pool. I'm probably going to stick with a sand filter and just add some DE after every backwash next season.
My vacuum hose isn't quite long enough... pool grew during the construction phase, and I had already purchased a hose (probably was too short anyway, but the extra length on the pool meant that I couldn't reach the far corner of the shallow end at all)... look at the dirt that had settled out after a week or so... this layer of dirt covered the entire pool... this is just a spot where I couldn't reach with the vacuum!
