The position of a river may also affect the percolation of water more upon one side of a valley than another, for a river seldom has its course in the centre of a vale, but is generally nearest to the steeper and higher side of a hill. The configuration of a country governs to a great extent the flow and quantity of the rainfall that sinks into the earth, as in a hilly country and in impervious soil the water is quickly discharged into an adjacent river, taking the easiest course. In a flat country and pervious soil rain percolates the earth and saturates the ground, or reappears in springs. As by drainage the retentive power of the soil is not allowed free operation, water rapidly flows into the drains instead of being chiefly held by the earth and watercourses, and channels are sometimes created; and where the rainfall is heavy or occurs in a comparatively short time floods may be caused, although the soil when drained, and therefore in a drier state, absorbs more water than when undrained and in a damp condition, water will pass through it quicker, and the discharge is thus increased in volume and velocity. Earth may be in a damp state, either from mere surface percolation and accumulation of water, or from springs which may never cease to flow; on the contrary, in rainless districts, from the almost perpetual daily drying power of the sun, the earth is sometimes found to be firmest and hardest upon the top, and for a few feet below it, than at greater depths. Separate masses of vegetation usually indicate damp places in a bare country. As rain flows more quickly from non-absorbent soil, such as rock, and slowly permeable earth, as the clays, than from porous soil, the surface discharge is greater; and unless the water is guided, pools are likely to be formed and weak places created, especially if the ground dips towards a cutting. It is well to remember that a cutting being excavated upon the side of a hill or upon table-land may change the direction of the flow of the drainage waters, and an embankment may obstruct and interfere with them, and should the strata incline towards the excavation, it obviously favours a discharge of water into it. If water permeates the soil or trickles down through fissures or veins, it will continue to do so until an impervious layer is reached, when it will be deflected and may become a current; therefore, whenever a permeable stratum overlies an impermeable, and the impermeable earth inclines, an increased flow may be expected, as also near the junction of tributary waters with streams or rivers.