Resumo. AIMS: The present study aimed at a morphological characterization of 18 lakes of the municipalities of Cidreira, Balneário Pinhal and Palmares do Sul, situated in the northern and middle coastal plain of Rio Grande do Sul, Brazil. METHODS: The morphometric survey was conducted in the summer of 2011/2012, based on echo sounder coupled with GPS and the application of remote sensing and GIS procedures. The morphometric parameters used for the characterization were: surface area, perimeter, perimeter development, volume, relative fetch, maximum depth, average depth, relative depth, median depth of volume, length and width. The statistical interpretation included Test of Normality, Coefficient of Variation, Correlation and Regression Analysis, Cluster Analysis, Discriminant Analysis and Principal Component Analysis. RESULTS: Eighteen lakes were described and five of them are presented by aerial photo and bathymetric map. They represent the proposed morphological lake types. The calculation of the coefficient of variation revealed larger amplitudes of parameters related to lake size than to depth. The application and interpretation of cluster analysis, confirmed by discriminant analysis, distinguished those lakes in five morphological groups, which differ mainly by size and subsequently by depth. The Principal Component Analysis identified the first component with positively charged variables of size and negatively charged relative depth, and a second component with positive charge of maximum and average depths. CONCLUSIONS: The low relative depths and the exposure to constant northeastern winds reduce the transparency of the lakes. Consequently, they are light limited by suspension of inorganic and organic matter. A property that limits the application of trophic state indices. The high dynamic of the water bodies produces an oxygenated sediment surface that facilitates the settlement by aerobic organisms all over the bottom area. The shallowness and the low median depths of volume are limiting factors for sustainable water supply management. The morphological characteristics of the coastal lakes make very difficult an accurate ecological assessment based on usual trophic state criterions and a sustainable water resource management using general guidelines.