Natural sediments can be made up of a varied mixture of mineral and organic material, reworked and stabilised by physical and biological processes. Knowledge of sedimentary processes is required for sediment transport models, which are essential for managing morphological change in coastal and estuarine environments. Many of these environments are dominated by mixtures of sand and mud. While good sediment bedform predictors are available for clean sands, there is limited knowledge of the behaviour of mixed sediments composed of cohesive mud and non-cohesive sand and this could limit the applicability of predictors based on clean sand. Recent laboratory experiments on cohesive and non-cohesive sediments, influenced by biogenic stabilisation, have shown that bedform dimensions decrease with increasing bed cohesive content. However, as bedforms developed and migrated along the bed, mud and biogenic polymers were winnowed from the bed into suspension. This thesis extends these laboratory results to assess the effect of cohesive material on sediment dynamics in a natural mixed sediment environment. To this end, fieldwork was carried out on tidal flats in the Dee estuary, UK, collecting measurements of the hydrodynamics, bed morphology and seabed properties, over a two week period covering a spring-neap cycle. It was found that the cohesive content of the bed varies with hydrodynamic forcing, and increasing bed cohesive clay content reduces bedform dimensions, effects the development of bedform plan morphology and reduces the rate of bedform migration and bed material transport. The amount of biological cohesive material (Extracellular Polymeric Substances, EPS) in the bed sediment was found to vary linearly with the cohesive clay content. Increasing bed cohesive clay reduced bedform dimensions and it would not be expected for bedforms to develop above 4 vol% cohesive clay content, in these field conditions. Bed material transport rate is reduced by increasing bed cohesive clay below about 3 vol%, and above this limit the bedforms appeared to stop migrating. Overall, sediment cohesion has the effect of making bedforms lower in height and shorter in length, and slows the bedform migration and material transport. Some current bedforms were seen to scale with water depth and were larger than expected for current ripples; these are described as transitional bedforms between ripples and dunes. As mixed cohesive sediment is found in many coastal areas worldwide, there is the potential for aspects of these results to have global relevance, if applied to regional models for sediment transport and coastal morphology.