Flow conditions and sediment transport capacity in steep mountain rivers are poorly known. One of the main problems is that the presence of macro-roughness elements, such as large relatively immobile boulders, have a strong influence. Experiments carried out in a steep laboratory flume, show that the bed surface occupied by boulders and their protrusion need to be taken into account for the estimation of sediment transport. The tests were performed with a wide mobile grain size distribution and a random position of the large boulders. A clear relationship between the dimensionless distance of boulders and sediment transport capacity is found. The bedload rate can be reduced by 60% when 15% of the bed surface is occupied by boulders, compared to transport rate without boulders. The boulder diameter also has an influence on sediment transport capacity. This is linked to both the surface occupied by the boulders and its average protrusion