Hydrology

Evaluation: Some Action Needed/Closely Monitor

Issue Summary: The availability of fresh water will become an increasingly important issue for all biological communities (including human) in the future. In the project area, the Coos Watershed Association gathers stream discharge data and monitors water use and land use.

Why do we care: Monitoring the condition of the watershed helps us manage the risk of flooding, fish habitat degradation, or water over-allocation.

Figure 1: Location of Coos Watershed Association stream gage stations in the project area


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What’s happening?

Figure 2. Sub-basins associated with the project area stream gages. Sub basin acreages are as follows: Big Creek: 3,411 acres; Winchester Creek: 4,109 acres; Elliot Creek: 1,018 acres; Three Mile Creek: 1,907 acres; Joe Ney Creek 634 acres

Precipitation
The primary source of precipitation in the project area is from rainfall. Snowfall only very rarely accumulates here. On average, the whole sub-basin receives about 64.5 inches per year (OCS, 2012). The precipitation intensity for a 2-year precipitation event is 2.8 inches in 24 hours (OWRD, 2012). Both of these values are used for statistical modeling of stream flow.

It’s worth noting that precipitation does not fall evenly in the project area. Most of the South Slough sub-basin receives between 65 and 63 inches of rain per year. On eastern slopes rainfall averages 61 inches per year. The relatively high topography in the project area in close proximity to the ocean can cause “orographic lifting” which occurs when a humid air mass is forced to rise in elevation, cool, and release its moisture in the form of precipitation. The resulting conditions usually cause a rain shadow in which precipitation falls during rain storms more on hillsides facing the storm and less on the wind-protected side. Most rain storms come from the Southwest which might explain why the Northeast portion of the project area is the driest.

Stream Flow
In the South Slough and Coastal Frontal project area there are currently five active stream gages. Two pre-existing gages, Big Creek and Winchester Creek, were reestablished recently, and three new gages were installed at Elliot Creek, Joe Ney Creek, and Three Mile Creek. Data from 2011 is available for all five gages. Additionally, data is available for Big Creek from 1983 Water Year (WY) to 1996 WY, and for Winchester Creek from 1992 WY to 1996 WY.

Figure 3.Discharge(cubic feet per second) at all stream five stream gages located in the South Slough and Coastal Frontal project area from December 15, 2010 through February 11, 2011.

Figure 3 displays the hydrographs for these stream gages for the period from December 15, 2010 to February 11, 2011. The hydrographs of Joe Ney, Elliot, and Three Mile gages have a much more rounded overall shape, which indicates a stream flow regime that is less “flashy” than either of the two larger creeks. Stream flashiness means that during storms stream flows rise quickly and then drop quickly suggesting abundant overland flow in the watershed which can lead quickly to erosion (see Land Use Effects on Hydrology below). Indeed, Big Creek experienced the greatest peak stream flow and appeared to be the flashiest of the streams evaluated, while Winchester Creek experienced the highest stream flow during periods of relatively low flow. The Joey Ney sub-basin, having the smallest drainage area (see Figure 2), experienced the smallest peak stream flow. Overall, the hydrology of the streams in the assessment area is very similar, because of similar soils, geology, land use, and precipitation patterns.

Table 1 shows the lowest recorded stream flow measurements at all five sites during the 2011 WY. These low flows all occurred in the summer and early fall, and can be used to represent baseflow summer conditions. Baseflow is an important factor to consider when evaluating stream health. A measure of the amount of sustainable flow in a stream channel, baseflow helps land and fisheries managers, and researchers determine the likely fate of aquatic organisms in streams. Stream flow above a base level of discharge is required to sustain fish and tiny aquatic organisms during prolonged dry periods (e.g, Oregon summers).The baseflow of Elliot, Joe Ney, and Three Mile Creeks is about 1.2 cubic feet per second (CFS). Big Creek experiences a somewhat larger baseflow, at about 1.4 CFS, and Winchester Creek’s baseflow is much larger—about 2.57 CFS.

Table 1.Baseflow values for gaged South Slough and Coastal Frontal streams.

Figure 3. Flow duration curve for Big Creek stream gage.
Figure 4. Flow duration curve for Winchester Creek stream gage.

Flow duration curves are a good way to compare the characteristics of stream flows between different stream systems. Figures3and 4 show flow duration curves for gages at Big Creek and Winchester Creek. The curves are almost identical up to 75 CFS, but the Big Creek gage peaks at about 320 CFS, while Winchester gage peaks at 140 CFS. Stream discharge values are directly related to the drainage area above the stream gage. Typically, the larger the drainage area, the greater the stream discharge values will be. However, even though the two stream systems have similarly sized drainage areas (Big Creek: 1,440hectares; Winchester Creek: 1,660 hectares), peak discharge for Big Creek is greater than peak discharge for Winchester Creek. The difference is most likely due to the relatively steep slopes of Big Creek. Steeper slopes means precipitation enters the streams more quickly through run-off, resulting in a “flashy” stream system..

Land Use Effects on Hydrology
When land is modified for uses like forestry, agriculture, roads, and industrial, residential or recreational development, watershed hydrology can be affected to varying degrees. We assessed the South Slough and Coastal Frontal streams and associated drainage areas for the effects of the various land uses found in those watersheds. We assessed in each drainage area the percentage of Hydrologic Soil Groups (HSG) present; the change in runoff rate; the percentage of forest roads; and the percentage of rural roads. Our results are summarized below and in Table 2.

The effect of such land uses on peak stream flow is of concern to landowners and land managers since severe increases in runoff can contribute to property and habitat damage from flooding and erosion, including landslides. The most important determinant for peak-flow increases is the ability of soils to absorb rainfall, which depends on land cover type and management treatments, as well as the characteristics of the soils (USDA NRCS 2007, OWEB, 1999).

The percentage of Hydrologic Soil Groups (HSG) in each sub-basin is described in Table 2. HSG are classified into four categories based on sand/clay composition and associated ability to retain water. All four HSG soil types are present in the project area. HSG Class A has the highest infiltration rate and is therefore associated with the lowest amount of runoff while HSG Class D has the slowest infiltration rate and therefore the highest runoff rates (See Table 2 HSG class definitions). Over a third of the soils in the South Sough watershed are classed as HSG D soils while less than 15% of the soils in the Coastal Frontal watersheds are similarly classed.

To calculate runoff risk associated with land uses, we compared the runoff rates of the project watersheds in their currently developed condition to the runoff rates of these areas in their pre-developed, or background conditions. This comparison provided us with a figure for change in the potential runoff rate between developed and undeveloped watershed conditions. Change in South Slough watershed runoff from background conditions increased by .71 inches, which is considered a moderate risk for peak-flow increases. In the Coastal Frontal watersheds change in runoff is estimated to be only 0.43 inches—low risk for peak-flow increases (see runoff risk level definitions in Table 2).

Lastly, we analyzed the percentage of forest and rural roads in the South Slough and Coastal Frontal watersheds for associated risk of peak-flow increases (Table 2), with 8% considered the threshold for high risk of altering peak-flow conditions.

Table 2.Land use effects on hydrology: Area the percentage of Hydrologic Soil Groups (HSG) present; the change in runoff rate; the percentage of forest roads; and the percentage of rural roads.
HSG Classes: A: <10% clay, >90% sand or gravel and low runoff potentialwhen thoroughly wet; B: 10-20% clay, 50-90% sand and moderately lowrunoff potential when thoroughly wet; C: 20-40% clay, <50% sand and moderately highrunoff potential when thoroughly wet; D: >40% clay, <50% sand and high runoff potentialwhen thoroughly wet. (USDA NRCS 2007)
Level of runoff risk definitions: <0.5” = Low Risk; 0.5 to 1.0” = Moderate Risk; >1.5” = High Risk (OWEB 1999)
Percent rural roads risk definitions: <4% = Low Risk; 4 to 8% = Moderate Risk; >8% = High Risk (OWEB 1999)

Water Uses
Table 3 displays the different types of water use in the assessment area based on surface water rights associated with each stream system (OWRD 2012a). The greatest consumptive uses in the South Slough watershed are in Joe Ney Creek, from which the Coos Bay North Bend Water Board has access to 23.4 cubic feet per second (CFS) for municipal water, including storage in the Joe Ney reservoir and others have rights for irrigation and agricultural uses; and Winchester Creek, from which the Coos Bay North Bend Water Board has access to 8 CFS for municipal water and Coos County has access to 0.7 CFS for irrigation. The greatest consumptive uses in the Coastal Frontal watersheds are in Big Creek from which private landowners have access to 2.43 CFS for irrigation, 0.18 CFS for storage and 0.1 CFS for domestic uses; Five Mile Creek from which private landowners have access to 2.21 CFS for water storage, and 0.01 CFS for domestic uses; and Cut Creek from which private landowners have access to 0.87 CFS for water storage, 0.03 CFS for irrigation 0.88 for agricultural uses.

Under the Instream Water Rights Act of 1987, the state of Oregon has worked on a voluntary basis with water right users, landowners, watershed councils, soil and water conservation districts, irrigation districts, and other organizations to acquire water rights on streams to ensure the conservation of adequate (base level) water supplies for fish and wildlife uses as well as human recreation and pollution abatement. In 1992, the Oregon Department of Fish and Wildlife established instreamwater rights in Winchester, Whiskey Run, Big, and Three Mile Creeks mainly to protect anadromous and resident fish rearing habitat.

Table 3.Water uses based on surface water rights data in the South Slough and CF watersheds.

Water Availability
Water availability was estimated using the Water Availability Report System (OWRD, 2012b). The project area has 9 water availability basins. The average discharge of water (in CFS) available was based on the 50 percent exceedance level which is defined as the percentage of time the observed discharge is greater than a baseline discharge value defined for the stream’s basin by stream gage data or modeling.

The net water available for the project area, shown in Table 4, is derived from subtracting the consumptive uses and reserved instream flow (established by instream water rights) from the estimated total stream flow. Net water availability is determined to be available year-round from only three basins in the project area: South Slough’s Elliot and Talbot creeks, and Coastal Frontal’s Five Mile Creek. All streams except these three creeks are determined to be over-allocated which means that should all water rights be exercised simultaneously, there is not enough flow in the creeks to accommodate all the uses. Winchester and Joe Ney Creeks have the greatest potential water deficit.

Table 4.Net water availability for the project area.

Stream Flow Restoration
In the Coastal Salmon Restoration Initiative (ODFW/OWRD 1997), loss of stream flow is identified as one of the three major factors exacerbating the decrease in fish populations (the other two main factors are fish resources and habitat). The Initiative ranked local streams by need and opportunity for stream flow restoration (Table 5), assigning highest priority to Winchester Creek, a Water Availability Basin (WAB)(Cooper 2002), an area in which an Oregon Department of Water Resources Water Availability calculation has been completed, and lowest priority to Whiskey Run. The other WABS considered high priority for restoration in the project area are Three Mile, Two Mile, Day, and Joe Ney Creek. Winchester Creek WAB was assigned highest priority because it has a consumptive use in the summer and fall of more than 10% of its natural stream flow.

Table 5.Local streams ranked by need and opportunity for stream flow restoration

Literature Cited
Coastal Salmon Restoration Initiative (ODFW/OWEB). 1997. The Oregon Plan, Restoring an Oregon legacy through cooperative efforts. Submitted to the National Marine Fisheries Services.160 State Capitol, Salem, Oregon. March 1997.

Cooper, R.M. 2002. Determining Surface Water Availability in Oregon. Oregon Department of Water Resources. Report #SW 02-002. 170 pp.

Greenberg, J. and K.F. Welch. 1998. Hydrologic Process Identification for Western Oregon.Prepared for Boise Cascade Corp., Boise, Idaho

Oregon Climate Service (OCS). 2012. Oregon Climate retrieved from http://www.ocs.orst.edu/

Oregon Watershed Enhancement Board (OWEB).1999. Oregon Watershed Assessment Manual.Prepared by Watershed Professionals Network. Salem. OR.

Oregon Water Resources Department (OWRD). 2012a. Water Right Information Searchretrieved from http://www.oregon.gov/owrd/pages/wr/wris.aspx

Oregon Department of Water Resources (OWRD).2012b. Water Availability Report System retrieved from http://map.wrd.state.or.us/apps/wr/wr_mapping/

US Department of AgricultureNatural Resources Conservation Service(USDA NRCS). 2007. National engineering handbook, Part 630 – Chapter 7- Hydrologic soil groups. USDA NRCS Report no. 210–VI–NEH. Washington, DC.

US Department of AgricultureSoil Conservation Service(USDA SCS). 1986. Urban Hydrology for Small Watersheds. Technical Release 55.