I've always thought that river flooding would always be the same at the same river stage. For example, if the river stage is at 20' flooding at location "x" would always be at the same depth. However, current flood stages along the Mississippi River are not correlating with past photos at the same flood stages in the past. I'm not aware of there being any major changes upstream that would accommodate the reduced volume we are now seeing. We are seeing roughly a foot difference (less flooding) along a 4-5 mile stretch of the river at the same location as little as 8 years ago. What other variables could result in different flood inundation at a given river stage?
Hydrology major here, for whatever that's worth :) Some possibilities from my readings/internships:
First I'd check the gage you're using for stage--if it's moved in the last 8 years, the stage may need to be correlated to what it would be at the former site. If it hasn't, it's more likely something changed in your stretch of channel.
If the channel has deepened at your location and not at wherever the gage is located, then you'll see less flooding. If the gage is upstream of your stretch of stream, I'd be suspicious of any new wetland areas that could accommodate water; if it's downstream, of any dam removal or other loss of channel restriction that could be allowing water to get past your site more quickly. Downstream changes in the channel could also affect your site! And if it's fairly urban, maybe the city has figured out ways to divert runoff farther downstream?