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Surface Water Availability in the Willamette Basin


At most times and in most places, surface water is abundant in the basin: in 1990, about 19,500 million gallons per day, equivalent to about 30,000 cubic feet per second, flowed daily, on average, from the mouth of the Willamette River, while daily withdrawals for out-of-stream uses were 870 million gallons per day, or 1,350 cubic feet per second. Scarcity does occur in some places during the summer, though, as natural streamflows fall and demands for water rise. Water typically remains abundant, however, below the basin's federal reservoirs, as releases from them can double or triple the summertime flows that would occur otherwise at the mouth of the Willamette. Water in the basin has many uses: to support fish and wildlife habitat, navigation, and recreation; to dilute wastes, generate hydropower, and irrigate crops; and to meet the diverse demands of municipal, industrial, commercial, and other users. Demands for surface water, however, are not expressed freely, as with most goods in a market economy. Rather, they are controlled by water rights, which specify the location and type of each allowed use, the amount to be used, and the priority date when the right was established. Although some water rights are intended to protect in-stream flows for fish and wildlife, most are for out-of-stream uses, such as irrigation, or for in-stream commercial uses, such as generation of hydropower. Roughly one-quarter of withdrawals in 1990 were intended for irrigating crops, 30% for public supplies, and the remainder for self-supplied domestic, commercial, and industrial uses.

boy drinking out of a water fountain

Oregon's system of water rights laws and regulations links each water right to others upstream and downstream. In effect, this system creates a queue of claims on water, with those water rights having earlier priority dates placed in the queue ahead of those with later ones, and the system allocates water along the queue, beginning at the front with the earliest water right and continuing until the available supply of water is exhausted. Transfers of water rights from the initial use to another, though allowed, have occurred infrequently and water rights established in the past for one type of use generally continue to be limited to that use. Thus, new demands might not receive a water right, if all the available water has already been allocated previously, and some existing water rights might represent demands that are not the same as they once were.

The Willamette River Basin Planning Atlas imagines the following possibilities for water availability by 2050:

Plan Trend 2050:  By 2050, increases in demand for irrigation dramatically reduce simulated streamflows in some parts of the northern half of the Basin. In some places "”Deep Creek and the Tualatin, Pudding, and Molalla Rivers -- the total supply of surface water is allocated to out-of-stream uses. Thus, these streams go completely dry in August, under dry-year conditions. No municipalities are adversely affected, but other out-of-stream demands (mainly irrigation) go unsatisfied in these subbasins. Map plant trend
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Development 2050:  Increases in irrigation have effects similar to those in Plan Trend 2050. Many streams run dry, as in Plan Trend, but, in general, streamflows do not fall as much as in Plan Trend because this scenario entails greater conversion of land from agricultural to urban and rural residential uses, and these types of development usually use less water, per acre, than irrigation. The pattern of increased scarcity resembles that in Plan Trend 2050. Map development
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Conservation 2050:  As in the other two water availability scenarios, increases in irrigation relative to 1990 reduce streamflows in some northern water availability basins, although none run completely dry in this scenario. The maintenance of streamflows in this scenario results partly from lower diversions per acre by irrigators and lower water use per capita in municipalities. Also important is the voluntary transfer of water from out-of-stream uses to in-stream flows. Some water availability basins, though, have lower streamflows in this scenario, compared to the other scenarios. The distribution of releases across the federal reservoirs changes, with some reservoirs releasing water earlier than they have done historically. Map conservation
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