The area is characterized by a fault zone system that has been partly sediment filled due to the retreat of the ice during the glacial period. The Sannäs fjord is following these fault lines as many other fjords on the swedish south west coast. The main geological features in the landscape consist of steep sloping Bohus granite, which is especially dominating in the outermost part of the Sannäs fjord. The sediment filled fractures consist of fine fractured clay, silt and morän. Some wave influenced sediments are also present in the region, such as gravel, sand and klapper. Fairly small areas are covered with peat and they are restricted to more elevated localities with some distance from the water channel (SGU, 2017). The vegetation is sparse in the outermost part of the fjord as well as in the archipelago and it is increasing deeper in the fjord system, where the conditions are more protective (Nordberg et al, 2012).

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The Sannäs fjord is approximately 7.5 km long, stretching in a NNW-SSE direction and is located approximately 25 km south of Strömstad. The width varies between 100-800 m and at its most narrow part of the fjord a bedrock sill  located with a depth of 8 m, the depth in the rest of the fjord ranges from 16-500 m. The marine sediments consist of clay or silty clay in the shallow bays, these are usually overgrown by eelgrass (Zostera marina) (Nordberg et al, 2012).