Streambed mobility in gravel-bed rivers is largely controlled by the rate at which particles with different grain sizes are recruited from the riverbed into the bed load. In this paper, we present a study case in which we explored this question, based on combining field observations using painted plots and the grain size analysis of a large flood sediment deposit in the River Esva, northwest Spain, and the generalized threshold model (GTM) competence model developed by Recking. The main aim was to accomplish a complete characterization of streambed mobility in this river. The obtained results suggest the large potential of the GTM model compared to previous competence models when searching for the quantitative description of particle entrainment and streambed mobility in the River Esva. We observed how the grain size of the bed load in the River Esva tended to be closer to that of the sub-armour bed material during large floods, while moderate magnitude flows tended to carry a relatively fine bed load. Additionally, we compared our results with previously published field observations on flow competence. This comparison outlined the large degree of site specificity in the links between grain size of the bed load and that of the bed material.

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<p>Erosion and deposition processes produce morphological forms over the channel and floodplains. Those forms depend on flow characteristics such as high velocity and deep water. By taking into account geomorphological criteria, the floodplain area presenting erosive and sedimentary morphologies is called Preferential Flow Area (ZFP). The ZFP was defined in the modification of the Hydraulic Public Domain Regulation (Royal Decree 9/2008, 11 January) and it is one of the units identified in the Spanish Flooding Zone Mapping System (SNCZI). The relationship between flood deposits and water flood deep is usually simulated by using physical models and hydraulic modelling. Nevertheless, there is little knowledge about the true relation by using field survey data. In order to improve the knowledge about the relations between geomorphological criteria based of flood forms and the ZFP, several field works were developed after floods in the lower part of the Sella River in Asturias (NW Spain). The present study considers the flood on 7 March 2009 with 5 year return period. 98 water flood data observed on field with 11 flood sand deposits thickness measured over floodplains was considered. The average value of sand deposits thickness was 0.14 m, with maximum data of 0.40 m, with an average value of water deep over these flood deposits of 1.54 m. An important correlation between these values was found and the linear and exponential model exhibit a good fitting explaining 81% and 86% of the deep-water variability respectively. By taking to account these results, flood sand deposits over 10 cm thickness could be used to identify representative areas of high flood hazard and the Preferential Flow Area.</p>

<p>Nalón River is one of the most important rivers draining the Northern watershed of Cantabrian Mountains (NW Iberian Peninsula). This drainage basin was subject to an intense coal mining activity during the 19th and 20th centuries. The resulting products of this mining activity (mainly those derived from coal washing) have been incorporated into the stratigraphic sequence of floodplains. This makes Nalón river a unique opportunity to study accretion rates in floodplains and to investigate the rate of sand sediment propagation and dispersion in gravel-bed rivers (two processes whose study is complex and elusive), as well as to study the spread of coal residues and other contaminants in the fluvial environment.<br>With these aims, we accomplished a stratigraphic survey of floodpain deposits in the lower alluvial plain of Nalón river. We identified a total number of 17 stratigraphic levels in a 3-m thick floodplain sequence. Some of these levels correspond to low-energy deposits (fine sand), while others correspond to high-stage flood deposits (very coarse sand and gravel), with abundant   sediment structures such as parallel lamination, cross-bedding and ripples. Samples taken from the floodplain sequence were prepared for grain-size analysis, organic petrography determinations (16 samples) and geochemical analysis (24 samples). Geochemical analysis measures the total concentrations for 63 elements by inductively coupled plasma-mass spectrometry (ICP-MS). ?). Additionally, we took 7 samples for luminescence (OSL) dating and one (small wood pieces) for 14C.<br>Coal is very abundant throughout the stratigraphic section, which suggests that sedimentation was simultaneous with the development of intensive mining that has developed in the last 150 years. On the other hand, there are several dams upstream the study section, the oldest one built in 1952. These dams have undoubtedly interrupted sediment continuity, so mining-derived sediments might not have represented a significant contribution to floodplain deposits after dam construction. However, the upper levels of the stratigraphic section contain abundant coal particles, which suggests that these levels pre-date dams (1952) or that the upper levels incorporated material derived from riverbed and bank erosion.<br>The objectives of this study are twofold: 1) to  analyse coal and  their associated  contaminants incorporated into fluvial sediments in the Nalón river; and 2) to reconstruct the sedimentary history of the alluvial plain, while exploring the use of coal and geochemical data as sediment markers.</p>