Surface Water is water that collects on the ground or in a stream, river, lake, wetland, or ocean.
However, we are not covering salt water at this point, so let’s continue with fresh water. Freshwater is naturally replenished by precipitation in the form of rain and snow and also from groundwater.
Surface Water carried in streams is often considered to consist of: a baseflow fraction which is made up of subsurface and ground water that infiltrates into the channel and a direct runoff fraction which enters the drainage system during and soon after precipitation and/or snowmelt periods.
The direct runoff presumably has had minimal to no residence time in the ground water reservoir and only short contact with soil or vegetation. Reactions in the soil/vegetation zone, however, are commonly extensive enough that the direct runoff has considerably higher chemical concentrations than the original rain or snow. Since the baseflow still has greater dissolved solids content, the solute concentration of stream water tends to be inversely related to flow rate. At very high flow rates, the water concentrations may be very dilute.
In addition, mixing of ground water and runoff, other natural factors that influence stream composition include reactions of water with mineral solids in the stream bed and in suspension, reactions among solutes, losses of water by evaporation, losses by transpiration from plants growing in and near the stream, and effects of water dwelling biota. This latter set of natural factors results in fluctuations of composition that bear little relation to discharge rates.