How long of a distance between water sources are we talking about? Forewarned, I'd rather carry extra water than a water filter. I can and have carried two days worth of water before.
About 48. West to east, it's about 32 from Colonial Creek to Washington Pass and another 16 to Mazama. As noted, it's literally all down hill to Mazama. Even if you are not a fearless descender, it's not going to take you long to cover that distance, so you don't have to ration water over the entire distance. Cross the river on Lost River Rd. and you will find a country store.
Second the concerns about stopping halfway up. Not only might there be snow on the ground, I wouldn't want to get stuck up there in a storm. It will also likely be very cold up there at night. The second time I crossed I ended up meeting a guy at the Witnthrop KOA who had stealth camped on the west side. He had started from west of Colonial Creek, started the climb late and was worried about running out of light. He said he melted snow for drinking water and froze his butt off. However, that was at the beginning of June.
Leaving from Colonial Creek, I wouldn't worry about daylight. If you were to start at 8 a.m. and take 10 hrs. to get to Washington Pass (at a 5 mph average, that would be 6 hrs. of pedalling and a generous 4 hrs. of stopping), it would only be 6 p.m. Another hour to Mazama and it's 7 p.m. In that neck of the woods at that time of year, it stays light well beyond then.
Finally, if the mileage works out for you, consider staying at Rockport the night before Colonial Creek. Howard Miller Steelhead Park has Adirondack shelters, which is nice if it's raining. (Reservations recommended, at least for weekends.) From there, it's a short, relatively easy day to Colonial Creek. Good rest for the legs before the next day. Not sure if there are groceries available in town. You could carry from Concrete. Marblemount, a little futher east, has more services.
