True that! Water sources, near and far (which is more than likely, closer than your origin - home ) within your backpacking area/route should be part of the plan. My troop and I backpacked 80+ miles for 10 days in the Bob Marshall Wilderness and plotted major water sources throughout ... and all I have was 20 oz nalgene bottle and my filtration system.
I usually take 2 1 quarts and a 3L camel back. That will last me 2 days if I'm not moving over 10 miles but I also carry a filter and refill at all opportunities.
While hunting the high country, water can be scarce up on the high ridges. Over the years I've learned that I need to have a couple of quarts with me, and a filtration system to fill 'em up again as the opportunity arises. I'll take some empty quart bottles or a collapsible gallon water bag/jug and fill them when I get to where I'm establishing my base camp.
I like the camel-back systems too. For a filter I use the little MSR/Sweetwater pump. It's proven very dependable and easy to use.
I use a Katadyn Hiker PRO Water Filter. It comes with a quick connect that hooks into your mounthpiece and pumps the water straight into the bag. Very easy and quick!
By the way, I recently switched out my old camelbacks for the new cabelas brand, which is a much stiffer plastic that doesn't get folded over and cut the water off, and there is NO plastic taste whatsoever. I usually fill the 3liter bag and carry a small plastic bottle. I do add some powdered gatorade/powerade mix after a couple days out just to break up the crickwater taste. They work good also to mask the iodine taste if that is what you are using.
No water for 5 to 7 days! A gallon weighs 8 pounds. I'd plan on toting an extra 56 pounds of water if you plan on doing any walking at all.
If you drink unfiltered water from a creek or stream you risk getting guardia. This is anywhere you have animals short of going to Antartica. The symptoms are upset stomach and the runs which will dehydrate you.
Bring a filter.
Where are you backpacking that has no access to filterable water? At least by day 6 you'll be down to your last gallon. Don't get lost! Dehydration will kill you fairly quickly and is touted as being very painful from those who have almost died from it. I'd never think of backpacking in an area that has no water. That is an expedition and you need a pack animal like a camel. Even a camel has to drink. I can easily go through a 32 oz Camelback in 6 hours of hiking or mountain biking.
I've had giardia, it sucks. Bring water, those people saying go without and drink from springs.... ask them after they had giardia, i got a host of problems from the medications and it didn't help my underlying stomach problems. Long story short, i missed nearly 8weeks of school.
I had/have an underlying illness that the giardia activated and i've had trouble with that for 5years now. The 8weeks was a mix of meds, giardia and my underlying condition. The doctors didn't know what the heck was going on and it was pretty bad. Morale of the story: don't get giardia.
Somewhat, the underlying condition never really settled back down. Don't take it personally but not really something I want to further discuss on the Internet where anyone can read it.
I dont' backpack hunt but I used to work construction. ON hot days (90 ish degrees) I can easily drink 4 quarts during the work day. ON cool ones, 20 oz will almost cut it. just food for thought.
I think the military did a study in the 70's where they came up with desert conditions marching. 1qt per man per hour if memory serves.