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. . . . . . . . . . . External Effects. Understanding the issue of external effects is w ~ t.Q U\ the key to comprehending both the nature of the salinity problems in the Colorado Basin and the alternative options for solving them by control measures. In order to see the dimensions of the external effects problem, let us utilize an economic model that focuses on irrigation. I Irrigated agriculture affects salt concentrations in the river more than any other man-induced activity. But the same analysis could be validly applied to any other activity that pollutes the river through salt loading or concen- trating effects. By irrigating, a farmer produces valua ble crops. But as a consequence of irrigation, salts already in the water are concentrated and natural salts contained in the soil are dissolved in the water, some of which passes through the root ,zones of plants and finds its way back to the river as return flow. This saline water is then utilized by other users down the river. Even if the salty irrigation water is somehow prevented from returning to the river, irrigation will have removed part of the water supply from the river, to a place where it is consumptively used by plants. The result may be to produce a concentrating effect (the river system will have picked up salt loads from non-point natural sources) so that downstream users will be using more salty water as a result of diminished flows. Thus, because of both natural processes and irrigation upstream, the salt concentration is higher downstream. The result is higher production costs for downstream users (in the case of irrigators, for example, yields may be reduced, or shifts must occur to more salt resistant, but profit-reducing 3 ~.L