I am just wondering if largemouths will use creeks and backwaters all year, or if they mostly just use them in spring and fall for heavy feeding and spawning.

Yes, Largemouth Bass will use River creeks all year long, and often much more than the river itself. Especially during spawn, bass will use the creeks for shallower bedding area due to the faster warming, and slower moving water. During fall, summer, and winter, some bass do move into the main river, but very many of them stay. There is abundant food in the creeks and backwaters, and provides more shelter from the much larger predators such as alligator gar.

Most areas of the country don't have allligator gar....so I have to disagree a bit with bassfisher14.

And MANY creeks are not large enough to support larger fish....some so small that an LMB couldn't make it up there.

For me....some of the BEST spots on most rivers will be where a creek flows into the river...especially if the creek water is cooler than the main river. But also during floods....or IF the flow of the creek is strong enough...they can create small scour holes in those junctions.

Pretty much depends on where you are in the country....the size of the creek...and the size of the river they are flowing into.

JMHO