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SECTION 1 - STREAM CORRIDORS Groundwater Losing Stream • Stream water table stream The top of the ground- water is called the -------` - ' --s - _,y 'x Watertavll?. - During dry periods, groundwater may RECEIVE water from the stream. Gaining Stream • Baseflow stage water table baseflow During wet periods, groundwater ADDS water to the stream As water moves through the hydrologic cycle, a significant portion infiltrates into the soil. It continues to move downward through the subsurface at a rate determined by the porosity of the soil particles. Sands and gravels, for example, have larger openings through which water can move faster than clays. Thus they are more permeable. When water reaches soils already filled with other water, it has reached the water table. It has moved from the unsatu- rated zone to the saturated zone. An aquifer is a geologically definable, saturated zone capable of producing significant quantities of water to a well. Groundwater moves from points of recharge to points of discharge- generally from places of high pressure to places of low pressure. Many streams in Colorado, particularly those moving from the mountains to the plains, pass through sand and gravel deposited long ago by ancestral rivers. These alluvial soils are highly permeable. If the adjacent water table is higher than the stream itself, groundwater is likely to be discharging into the stream. This is called a gaining stream. If the adjacent and underlying water table is lower than the stream, it is likely to be a losing stream. That is, water from the stream will move into the alluvial soils. Irrigation of lands near streams raises the water table. Groundwater pumping lowers the water table. Both activities can affect the amount of water flowing in the stream. Residues of chemical substances deposited on lands can infiltrate with water and collect in saturated zones. High concentrations of these substances can make groundwater unusable. In some locations the pathways for contaminated groundwater may lead into surface water such as streams. 12