Should I understand a water source to mean a spring/well, or any place
where hikers may collect water (streams, lakes, etc.)?
Yes. Any source of water - no matter how large or small - should be avoided when choosing a camp site. 100 meters is just a guideline, 200 meters is better. 200 meters and out of sight is great.
The reasons are several-fold:
- Waste - wastewater should never be put in creeks/lakes/etc - and although many people will walk 200 meters away to dispose of it properly, most wont. The closer people are to a body of water, the more likely they are to say "ah to heck with it" and dump it in. It's difficult to resist the temptation to just do a quick rinse of hands, plates, etc in a creek that is right next to you.
- Erosion - The riparian areas next to creeks and lakes are often the most sensitive, and can not handle large amounts of traffic. Traffic up and down and along the creek banks can cause rapid erosion of soil and cause vegetation loss. This is a problem from alpine to canyon ecosystems.
- Visual Disturbance to other hikers - Setting up your tent beside that beautiful pristine lake is an awesome scene, right up until that group across the lake sets their tent up in your view. Don't be 'that guy.' I've successfully distributed 15 groups of backpackers around a high-alpine lake so that every single one of them could pretend they were the only ones out there.
- Disturb/Encounter wildlife - while you are asleep, chances are there is a whole host of thirsty critters strolling up and down the creek / lake shore looking for drink (or to eat those drinking). These are also natural travel corridors. Your presence can be disruptive to the fauna, or could put you in the path of grumpy wildlife (bears).
These reasons apply to all ecosystems (apline, desert, grassland, etc) and as such, camping away from water sources is good 'Leave No Trace' practice in all situations.