Google Maps

I might have better luck asking this in a progamming group.
But I'll start here anyway.

If you live in a place like the Gallatin Valley, in Montana, that has
a large
number of spring creeks, and if
you like to fish, you want to know where they are. One way to look
is to fly around the valley at low altitude and look. But the springs
are hard to distinguish from meandering river channels most of the
year.

Except at runnoff time, and in the dead of winter. In winter the
springs aren't frozen, so they stand out, even from 4000 feet.
At runnoff time the rivers are chocolate brown, while the springs
appear jet black from above. But flying is expensive, and not
always an option (I found a new spring 2 days ago!).

So I thought about Google Maps. But their images are sometimes
a year old, and I don't see how to request June 1 images.
Anybody know of any other satellite image archives, where you
could look at images taken at specific times, so you could spend
a few hours at the keyboard, and find all the secret spring creeks
in the State?