WATER RESOURCES

Surface Water

The average annual precipitation in the Willamette Basin is 63 inches. This results in a volume of more than 40 million acre feet of water falling on the basin each year. In Marion County, the annual rainfall ranges from 40 inches in the Salem area to over 130 inches per year in some parts of the Willamette National Forest.

The eastern border of the County reaches the crest of the Cascade Mountains. This area of the County receives the greatest amount of rainfall and runoff here forms the headwaters of a complex stream and river system. These streams make their way into one of three major sub-basins of the Willamette Drainage Basin. These three basins are:

1.    The Clackamas River Basin, which includes only a small area in the northeastern portion of the County;

2.    The Santiam River Basin, which forms the southern border of the County;

3.    The Pudding River Basin, which runs parallel to the Willamette and joins it just north of Aurora.

A map of the Middle Willamette Drainage Basin that includes the three basins was prepared by the State Water Resources Department in 1977. This map is available for review in the Planning Department office and it illustrates all of the significant surface water features in the County.

The yields of Cascade streams reflect the impact of winter precipitation, occurring largely as snowfall. The average flow increases through the fall months, peaks initially in December, maintains a high flow through May and then subsides gradually.

Flooding is of obvious concern during the winter months. The danger of flooding along the Santiam and Willamette Rivers has been significantly reduced due to upstream control of flow conditions. Peak runoff can be stored in Detroit Lake and other reservoirs along tributaries of the Willamette and the water can later be released when the threat of flooding has subsided.

Drainage is a problem in flat-lying areas of the Willamette Valley portion of the Pudding Sub-basin, where the many areas identified in the Soils Survey for Marion County need artificial drainage. Large-scale irrigation is likely to aggravate this problem.

The marked decline in stream flow during the summer months is of critical concern to water quality management planning. Low stream flows are associated with high water temperatures, relatively high concentrations of nutrients, low dissolved oxygen and algae growth. These conditions are harmful to fish and would reduce their populations significantly if allowed to continue. Upstream reservoirs augment diminished flows to maintain a minimum supply of water for fish and wildlife, recreation, irrigation, municipal uses and the assimilation of waste effluents released from sewage treatment plants.

In cooperation with the Department of Environmental Quality, the Mid-Willamette Valley Council of Governments has undertaken a study of water quality management and water pollution control (see pp. II H-3 in the Marion County Comprehensive Plan). Data has been incorporated into a technical study to help define the role that population growth, economic development, and related land uses play in analyzing local and regional water quality problems. The data will also be used to help ongoing and anticipated facility planning efforts into an area wide planning philosophy to achieve cost effective solutions to local and regional water quality problems.

Intensive forest management practices and soil conservation techniques are essential to maintain high water quality downstream. Clear-cut lands must be quickly restored to prevent erosion, flooding and excessive reservoir siltation. As streams flow through forested areas and on downstream, the water should remain cool, clear, free of sediment and debris and low in biological contamination to meet municipal, recreation and anadromous fishery demands. Agricultural lands, especially in the foothills areas of the County, should also be managed in a way that will maintain adequate surface water and groundwater quality.

Groundwater

Several groundwater studies have been completed for portions of Marion County by the U.S. Geological Survey and the State of Oregon. An additional report prepared by the Pacific Northwest River Basins Commission serves as a summary of the information available for the County. This report indicates that the quantity and quality of the groundwater resource in the County is generally satisfactory to excellent. Some potential problem areas are discussed in this report but specific information relating to their extent and possible solutions are not identified. The following paragraphs describe the current groundwater conditions for each of the three sub-basins in the County.

In French Prairie, groundwater in storage within 200 feet of the surface is estimated to be about three million acre feet. In the same area, it is estimated that annual replenishment to the aquifer is about 160,000 acre feet; eight times the present rate of pumping. For most of the Pudding Sub-basin, no evidence of over development, such as declining water levels, has been noted. For the valley area, withdrawals are much less than estimates of recharge; therefore, pumpage in the sub-basin could be increased several times.

Groundwater is abundant in the Willamette Valley portion of the Pudding Sub-basin with small to moderate quantities in the foothills and small quantities in the mountains. Groundwater is used for irrigation of nearly 34,000 acres; for the municipal supply of 14 communities with a combined population of more than 17,000; and for the domestic supply of most of the rural and suburban homes in the sub-basin.

The Pudding Sub-basin has some of the best groundwater reservoirs in the Willamette Basin, but it also has a number of groundwater problems. In some parts of the foothills and mountains, groundwater at shallow depths is not adequate for domestic and stock supplies. Mutual interference between wells is a problem in several areas of concentrated pumping near Woodburn. In the foothills north of Stayton and Turner, where a number of irrigation wells pump large quantities of water from the basalt aquifer, water levels have been declining for several years, which indicate that the aquifer is locally overdeveloped.

There is generous supply of groundwater in the Santiam Sub-basin. More than 1,000 wells supply water to irrigate about 27,000 acres. In a small area in the loop of the river southeast of Jefferson, yields of irrigation wells are reported to decline during pumping season. Part of the decreased yield can be traced to deterioration of the wells and does not seem to be due to overdevelopment.

Groundwater is abundant in that portion of the Clackamas Sub-basin which is included in Marion County. The entire area, however, is within the National Forest and it is doubtful that the water will ever be used extensively.

Municipal Water Systems

The Pudding and Santiam River basins represent the two basic sources of water for all of the municipalities in Marion County. Wells are the primary method for obtaining water in the Pudding River Basin. Of the 11 cities in the basin, only two rely on surface water. Scotts Mills draws on Butte Creek for supplemental supply in the summer months and Silverton takes water from Abiqua and Silver Creeks. Surface water is the main source of supply for those municipalities in the Santiam River basin. The cities of Detroit and Idanha obtain water from tributaries of the North Santiam River. The City of Jefferson is the only city in the basin relying totally on a well. All other cities, including Salem, divert water from the North Santiam River. Available information from the cities, State Department of Water Resources, State Department of Environmental Quality, and State Health Division, does not indicate any water quality problems that are related to land use and development in the County. The State Health Division has indicated that Scotts Mills, Idanha, Detroit and Gates have experienced some water quality problems but they appear to be related to improper treatment, not poor water supply. Water service is a primary public facility for each city and the status and planning for each is included in the respective Comprehensive Plan. There are no apparent land use conflicts associated with any city well systems. Potential land use conflicts with surface water systems for each waterway are discussed below:

Butte Creek

This watershed encompasses portions of Marion and Clackamas Counties. Scotts Mills is the only city in Marion County that utilizes water from the Creek. A spring and two wells represent the primary source of supply and Butte Creek is only used on a temporary basis to meet summer peak demands. The entire watershed is zoned for resource use (FT, Farm/Timber, and TC, Timber Conservation, in Marion County and EFU-20, Exclusive Farm Use and TT-20, Transitional Timber, in Clackamas County).

Abiqua and Silver Creeks

These two adjacent watersheds provide water for the City of Silverton. Water is diverted from Silver Creek Reservoir and Abiqua Creek southeast of the intersection of Timber Trail and South Abiqua Road. Both watersheds are predominantly zoned for resource use (FT, TC, and EFU). Approximately 790 acres of land is zoned AR (Acreage Residential) which allows for the creation of rural homesites that are generally 1.5 to 3.0 acres in size. An additional 210 acres is zoned AR5 (Acreage Residential 5 acre minimum). The areas zoned AR recognize existing development and although additional homesites can be expected, the potential for significant numbers of new residences is limited.

North Santiam River and Little North Fork

The North Santiam watershed is divided almost equally between Marion and Linn Counties. The portion of the basin in Linn County is primarily designated and zoned for agricultural and forestry use. In addition, over half of the watershed is under the control of the Willamette National Forest and the Bureau of Land Management.

Salem, Stayton, Lyons (Mehama), Mill City, and Gates all utilize water from the North Santiam River for domestic use. None of these municipalities divert water directly from the Little North Fork but it is a tributary of major importance for Salem and Stayton. The majority of these watersheds are zoned for resource use (SA, EFU, FT, TC) but there are six exception areas, which contain a combination of properties that are zoned AR, P (Public), CR (Commercial Retail), and IH (Heavy Industrial). The P, CR, and IH zones apply to existing commercial and residential uses and two timber related industries. The 750 acres designated AR also include some properties that are committed but not fully developed. However, the amount of land available for additional homesites is minimal.

Detroit and Idanha are also within the basin but the water resources are tributaries of the North Santiam River. Mackey Creek and Breitenbush River are tapped by Detroit and virtually all of the land included in the two drainage basins is managed by the U.S. Forest Service. Idanha is suppled by Rainbow Creek, a spring, and Taylor Creek, which are located in Linn County. These tributaries are within the Linn County Forest Conservation and Management zone and most of the land involved is within the National Forest.

There are no formal agreements between different jurisdictions in the County related to surface water quality and quantity. However, the County and the U.S. Forest Service have taken steps to reduce the likelihood of detrimental water quality and quantity impacts. As mentioned above, the existing zoning provides water resource protection. Also, the cities of Silverton and Salem, which utilize the Abiqua Creek, Silver Creek, and the North Santiam Watersheds, have reviewed the County zoning designations and have voiced no objection to the permitted uses and land division policies in those watersheds. Theses cities will be notified of any proposed conditional use that may have an impact upon the water resource. The cities in the North Santiam Watershed are informed of U.S. Forest Service land management plans including timber cutting that are proposed on National Forest lands that could have an impact upon a municipal water resource.