The northern half of Wales has numerous lakes and other places suitable for swimming. Generally, swimming is allowed in most natural lakes, but not in reservoirs. Lakes can be cold and very deep - especially if high up in the mountains, so take care when swimming. Welsh lakes can be classed as either oligotrophic or mesotrophic. Oligotrophic lakes are low in nutrients, and therefore have little or no algal growth. This makes entry easier as the bottom isn't slippery. Such lakes are usually found high in the mountains. Mesotrophic lakes are found lower in the valleys, and often have considerable algae growth on the bottom, as well as various water-weeds.