Contrasting patterns of wood storage in mountain watercourses narrower and wider than the height of riparian trees

•In stream, spatial density of wood decreased nonlinearly as channel width increased.•In river, spatial density of wood increased with increasing channel width.•Patterns of wood storage depend on the ratio of channel width to riparian tree height.•Relation between total and specific wood storage differs between stream and river.

Large wood exerts a beneficial influence on the functioning of riverine and riparian ecosystems but can be also the source of flood risk. Several studies have recognised a trend of decreasing amounts of large wood on unit channel area with increasing width of mountain streams. This study verifies whether this trend can also be generalised for wide mountain rivers. Longitudinal wood distribution was compared for two watercourses in the Polish Carpathians: second- to fourth-order Kamienica Stream, 14 m wide on average, and the fifth-order Czarny Dunajec River with a mean width of 52 m. In the stream, both the number and the mean mass of wood deposits were unrelated to channel width, and this was reflected in a lack of the relationship between total wood storage (the amount of stored wood per channel length) and channel width. In turn, specific wood storage (the amount of stored wood per channel area) decreased nonlinearly with increasing channel width. In the wide Czarny Dunajec, the number and the mean mass of wood deposits increased as the river widened, and this was reflected in a marked trend of increasing total wood storage with increasing river width. Here, the width-related variation in total wood storage was so high that it overcame the influence of increasing channel area on calculated values of specific wood storage, which also increased with increasing river width. This study shows that different mechanisms known to govern large wood retention in the channels narrower and wider than the height of riparian trees are reflected in the contrasting patterns of wood storage in mountain watercourses of different relative width. Different relations between the tendencies of total and specific wood storage in the two types of channels emphasise the need of identifying the spatial density and the total amount of large wood.