this is because they are also fed by groundwater.if enough rain falls then water will eventually pass through the soil and reach the rock beneath.what happens next depends on the type of rock.if the rock is made up of small grains,like sandstone,this allows the water to pass through.if the rock is made up of small grains,like sandstone,this allows the water to pass through.these 'permeable'rocks can store large amounts of water.for instance one cubic meter of sandstone can hold upwards of 200 litres of water.other types of rock that are not permeable may have cracks,which allow the water to travel through the rock until it reaches a point where there is on more space for water.this is called the water table and the water thal moves around below it is called groundwater.