The river networks fulfull one other function, also: in recent decades, some of the areas that earlier were dammed for use as flour mills and sawmills, have been taken over for use in the water-consuming aquaculture industry of smolt production. Other rivers have been dammed up for use as drinking-water reservoirs. Some are situated such that they collect agricultural run-off from and sewage seepage from from human settlement. We also know that in earlier times the water sheds were often used as dumping grounds.
On Sotra, one finds as many as 50 smaller river networks, most of them having several smaller lakes. Only five of these have a catchment area of over 5 km², while over half have a catchment area of under 2 km². The majority of these are far away from settlements and agriculture.
The Fjell river network is the biggest water system on Sotra, with a total of 10 lakes of various sizes. The upper parts of this network are nutrient-depleted and a bit unclean. The lower parts got contaminated with nutrients, organic material and intestinal bacteria in the past. This was the case with Lake Eikhammarvatnet, especially. But, conditions have improved: calcium carbonate (chalk) was added early in the 1990s to neutralize the acidity, and the establishment of a sewage treatment plant at Lake Eikhammarvatnet made the whole Fjell river network healthier than it had been for a long time.
There are good conditions for both spawning and growth of baby sea trout in the river upstream at Lake Kolavatnet. The Sotra Sportsfishermen group administers fishing rights in the water system. Here, a lot of fine sea trout get caught each year.
