Drinking water biofilms and deposits represent organo-mineral complexes associated with a diversified microbial community. Such highly hydrated organic structures are made of cells (about 106 to 107 cells/cm2) embedded in a matrix of extracellular polymeric substances, EPS, carrying some metals and carbonate. Hetero-glycoconjugates are generally thought to form the EPS matrix, but there is no characterization of this low EPS mass in drinking water biofilms. Biofilms and deposits accumulated on drinking water pipe surfaces amount to about 50 μg organic carbon/cm2. Bacterial cells do not contribute to more than 5% of the deposits, and by contrast the rest of the organic material (EPS matrix, humic substances, etc.) is predominant. Two approaches were used to assess some properties of this little-known matrix. First, the presence of hydrophobic domains within the EPS matrix was demonstrated on laboratory-grown biofilm labeled with amphiphilic functionalized quantum dots. Second, thanks to a theoretical approach, we found that young drinking water biofilms behave like a viscoelastic solid with a low EPS entanglement. AFM imaging performed on young drinking water biofilms revealed a very large polydispersity in the cluster size distribution (from few micrometers to more than 20 μm in equivalent diameter). Additionally, by generating a tangential shear stress, which scraped or displaced the biofilm aggregates, we obtained consecutive AFM imaging that can be conceptualized as removing the biofilm materials ‘layer by layer’. Indeed, each layer was composed of biofilm materials showing similar cohesiveness or viscoelastic behavior. On the contrary, within the same biofilm a different case was noticed where the biofilm height did not change significantly in spite of an increasing shear stress. This suggested the presence of rigid material at this location. Interestingly, these two opposite behaviors found within the same biofilm, imply the presence of heterogeneous bacterial clusters. Such a stratified organization was also indirectly demonstrated by submitting drinking water biofilms to a fast increase of hydrodynamic forces and/or shock chlorination. Hydrodynamic strength increase permitted to remove a fraction of the attached biomass but small cohesive aggregates (< 200 μm3) eluded to those wall shear forces. High chlorination induced a release of deposits, and strong intracellular damage, but many cells (dead cells for most of them) were still adhering to the surface after the treatment. The cohesiveness of aggregates forming drinking water biofilms as well as their resistance to hydrodynamic shear forces or oxidation will be discussed.