What is the spaitial resoultion of your DEM. May be it is because you are using low resolution DEM in nearly very flat topography. And the river bed in the DEM may be showing the same as Floodplain mean very flat almost non-identifyable.The same happened with me. You try to use a higher resolution DEM then check what happens.

Interesting following answer.....

River slope is difficult to estimate, with standard DEM resolution, and even with high resolution LiDAR DEM. At best, an estimate can be made from valley slope divided by channel sinuosity. Actually measuring it, not that easy either, as I remember, it was water level at top of riffle to water level to another top of riffle, divided by the channel distance between them. For relatively low gradient stream types, survey grade elevation measures are taken. You might also use elevation top of point bar to top of point bar, divided by channel distance between. If you have noticed, many topographic maps do a poor job of georeferencing streams, especially in low gradient coastal plain. You may find the actual location of channel topographic contours to also be off in location.

One might calculate channel slope from measurements taken on site, but the best one might do with DEM and HEC tools without a proper survey is an estimate of channel slope. If one were to use the flat terrace or floodplain adjacent to the channel divided by channel length, an estimate (ball park) of average slope could approximate.

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Is it actually an error or a warning? Most likely z unit value for the projection you are using is not defined!

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Go to "Geo-processing--> geo-processing options --> unclick background processing. "

Then re-compute river slope

This should work fine

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thanks for all,giving their views for solving my problems,