The beds of the rivers are generally composed of clean, compacted gravel and flints, which provide good spawning grounds for Salmonidae fish species.[3] Since they are primarily fed by aquifers, the flow rate, mineral content and temperature range of chalk streams shows less seasonal variation than other rivers.[3][4] They are mildly alkaline[5] and contain high levels of nitrate, phosphate, potassium and silicate.[3] In addition to algae and diatoms, the streams provide a suitable habitat for macrophytes (including water crowfoot)[6] and oxygen levels are generally supportive of coarse fish populations.