ABSTRACT Seasonal dynamics and storage of particulate organic matter (POM) were examined at three sites in adjacent watersheds in the French granitic Massif Central mountains. The three study areas differed mainly by their streamside vegetation and morphology:

-an undisturbed site in a deciduous forest located in a V-shaped moderately incised valley,

-a site in a pasture area with narrow forested buffer strips along the banks,

-a site in a pasture with only isolated trees.

The last two streams flow through gently sloping plateau valleys.

For each season between July 1991 and April 1992, the freezing-core technique was used to extract three cores from different morphodynamic units at each site.

Riparian vegetation influenced the seasonal dynamics of POM inputs. Streambank vegetation and macrophytes (also influenced by the canopy) were sources of organic matter and they controlled its transport by modifying the retentiveness of the channel. The amount of POM buried in the bed sediment depended on interactive factors such as substratum composition and porosity. Porosity was closely linked with the percentage of grain size <1 mm that acts as a limiting factor. The duration of POM stored within bed sediment depended on the timing and magnitude of storms and on retention structures. During high discharge, channel and bank morphology (percentage of riffles or backwaters, slope of the bank) and streambank vegetation influenced transport and retention of POM.