Assessment of contemporary state of a fluvial system requires recognition of a relationship between transport capacity, channel morphology and sedimentological properties of a bedload and its hydraulic effect. The objectives of study are: (1) to describe relationship  between pebble shape and orientation, and cluster bedforms visible on a surface of emerged channel bars, and (2) to provide an answer on question, whither textural properties of gravel bedload can help estimate channel stability, and dynamics of sedimentary environment (in a given channel reach). The measurements we done in a few chosen sections of two mountain streams – Rycerka and Danielka, in Beskid Żywiecki (South Poland). On a surface of emerged channel bars, at first was made photographic documentation of pebble cluster, and then pebble orientation and shape was measured in fractions: 30–50, 70–100, 150–170 and 200–250 mm. The bed material is composed of a range of oblate and prolate gravel size particles. Sparse boulders and cobble size material usually prevent its orientation form the transportation phase and remain immobile during most flows (or smaller flood events, also). These particles initiate development of other sedimentary structures, i.e. obstacle clasts. The orientation of particles with a-axis transverse to flow is associated with traction carpet and indicates a high bedload transport rate and quite abrupt deposition. Particles of 70–100 mm were imbricated (a-axis parallel to flow). During transport, that particles move by sliding or in saltation, but even slight decrease of flow can cause its settling, and reorientation. Typical imbrication developed on a side of channel bars. Unclustered bedforms (i.e. open bed non-interlocked) were located in middle and distal part of channel bars and build with finer particles: pebbles and granules. Their orientation was plane or chaotic. Due to the low flow we can assume that they prevent its position from transportation phase. This study showed that orientation of particles and their arrangement in characteristic sedimentary structures is highly dependent on lithology of bed material. Flat and elongated pebbles (abundant in investigated streams) are most susceptible to form imbrication, and clustered bedforms, like: obstacle clasts or complex clusters. Such sedimentary structures can develop very quickly, and hence increase stability of gravel-bed stream.

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