In southern Italy, especially in Calabria, rivers are dry for most of the time, but intense rainfalls may turn them into roaring monsters, causing devastating floods. These rivers, locally known as “fiumaras”, are poorly studied though they play an important role in landscape shaping and pose serious threats to the local infrastructures and urban settlements. Basic catchment and channel geomorphic data of several rivers were collected from the literature and in the field. A comparison is made with river catchments of similar size in more humid environments to demonstrate that local physiography, watershed geomorphology, and channel characteristics may exacerbate the risk of flooding. The sedimentology of the study rivers is investigated to verify if fiumaras have specific bedform associations or stratigraphic arrangements that can be used to interpret ancient sandstones and conglomerates as deposited in an active tectonic setting under the Mediterranean climate. Four representative rivers were selected for investigation on the alluvium architecture, and field campaigns were carried out to collect bed material samples and to identify the occurrence of coarse and fine-grained bedforms. The fiumaras have a braided morphology, but the longitudinal bars do not have a fine tail and result from dissection processes rather than large bedform deposition and downstream migration. The braid bar characteristics, the poor internal organization of the tabular beds, and the occurrence of the largest boulders on top of the bars indicate the prevalence of high deposition rates from hyperconcentrated flows.