A word of caution might be supported somewhere in the software background. Raster based DEMs are typically developed from topographic maps, and often lack the level of detail needed to properly define the channel cross section and as well define if the stream type is entrenched such as a gully, with no floodplain. The flat areas adjacent to rivers and streams may be active floodplains or inactive floodplains (terraces). With some history of flooding, field level indicators, soil sampling, location and elevation of point bars, or channel morphology measured, one can often confirm if the flat areas adjacent to waterways are floodplains. LiDAR is another helpful tool that can help reduce the raster pixel size and increase level of detail, such as defining bank entrenchment, or whether channel is braided indicating potential for frequent flooding and possibly channel instability and adjustments if not anastomosed.