The US Geological Survey is constantly collecting data about streams, their flows, heights, temperature, etc.
They also have a website where you can view this data, as well as select which data you would like to view, and over what period you'd like to see it graphed.
I don't know how useful this may or may not be to cold water fly fishers, but it is an interesting resource to have on hand.
The official link
: USGS Real-Time Water Data for the Nation
You can select your state, and then select which area you would like to view. You can also enter GPS coordinates, site number, or search by name.
Here is an example of the Manistee River, in Michigan near Mesick:
Real-time Data - Manistee River Near Mesick
You can see the various data, plotted over a 7-day period (or more, if you specify differently).
How temperature is changing, and trends over time, might be something useful.
They also have a tutorial on how to use this site, if you need more help:
I don't know if this has been posted before, so I thought I'd post it and maybe it might help some of you.