Even with low relief landscape, local channel sections could have falls and cascades due to some geologic change or channel constriction or control that forces water through an entrenched channel with steeper gradient and exceeding threshold for critical flow. If the channel exhibits standing waves or other critical flow indicators, but is low gradient, then bedrock or similar control is likely producing a cascade looking flow regime. But normally, cascades are not low gradient. If you were in coastal plain, you have low gradient conditions in general, but the transition from one marine terrace to next terrace can be steep below scarp, producing steeper gradient, perhaps gullying in fine grained substrates common to marine terraces. So I would probably look for a geological explanation. Releases from lakes or dams can in some instances produce excessive flows for constrained local channel dimensions that have cascade like appearance. As I remember Mongomery and Buffington inventory, you would separate the cascade section as a separate habitat type from other non-cascade sections. In the unusual circumstance where there is an abrupt increase in flow from karst geology with high spring flow, this is another circumstance that might produce something that looks like a cascade. If the inventory section has critical flow indicators but lacks the gradient, I would still inventory as a cascade section and earmark the variance for further analysis if needed.

Prof Hansen  thanks alot for the response. As much as I don't want to over assume,  my study area is within humid trophics, lithological it is heterogeneous comprising of older granites and metasediments, it's a low relief catchment with no glacial history. 

I wouldn't mind having your email sir.