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Hypothesis #5: Alteration of riparian stands has increased stream warming in some reaches of the Ten Mile River, even where direct shade has been retained.

Supporting Evidence from the Literature

Substantial current scientific literature suggests that alteration of the riparian canopy, even well back from the stream, can open air flow and change the microclimate over the stream (Brosofske et al., 1999). Chen (1991) found that the riparian zone of influence was over two site potential tree heights back into the forest and that riparian thermal buffering functions decreased substantially under one site potential tree height (see chart). Increasing airflow, particularly in areas with high summer air temperatures, can increase heat exchange with the stream and thereby elevate water temperatures (Bartholow, 1989; Poole and Berman, 1999). For a full discussion of factors influencing stream warming, please see the Temperature Background page. See also Vegetation Types for information on how regional remote sensing imagery can be used to analyze riparian conditions (Warbington et al., 1998) in reaches with different temperature regimes. 

From the Ten Mile Basin

Several monitoring reports from the Hawthorne Timber Company (HTC), and its predecessor Georgia Pacific (GP), have sections devoted to water temperature (Ambrose et al., 1996; Ambrose and Hines, 1997 and 1998). These reports note that mainstem reaches of the Ten Mile River are unsuitable for coho salmon due to high water temperatures in some years (Ambrose and Hines, 1997). They offer a hypotheses that these reaches were "naturally" warm due to stream width and also that they were never important coho rearing reaches. The latter is countered by the California Department of Fish and Game (1961), which documented coho using lower reaches of the South Fork Ten Mile River.  Historic conditions in for these mainstem reaches would have included considerable shade and microclimate influence from  200-300 foot redwood trees,  narrower average channel width as a function of less sediment delivery and storage, and input of cooler water from tributaries with older riparian forests.

The U.S. Forest Service has developed a California-wide vegetation map derived from 1994 Landsat imagery, which includes the Mendocino coast and the Ten Mile River basin (Warbington et al., 1998; Schwind, 1999). The imagery provides a way to view forest stand conditions, at the one hectare scale. These data were used in the KRIS Ten Mile Map project to characterize tree size over the landscape and as a riparian analysis tool when applied to a 90 meter riparian buffer strip on each side of the channel. These data show that much of the riparian community of the mainstem Ten Mile River is dominated by early seral forests likely to allow increased airflow over the stream. A tiled aerial mosaic of the Ten Mile River basin in 1999, provided by Matthews (2000), confirms that the canopy over mainstem reaches is open and that the riparian zone contains few mature conifers.

Water temperature data from the Ten Mile River basin show middle and lower reaches of the South Fork and the Middle Fork are too warm to be suitable for coho salmon in most years. At the upper South Fork temperature monitoring station above Redwood Creek (SFT6) such water temperatures appear to be the result of alteration of riparian stands.  Temperature probe locations 2.0 km upstream of this location, at Gulch 11 (SFT11) and the upper South Fork (SFT12), are cool enough to support coho salmon juveniles.  This reach is an example of where canopy and riparian conditions seem responsible for the elevation of water temperatures. California Department of Fish and Game (CDFG, 1961) indicated that this reach harbored coho salmon but HTC fish monitoring data show that coho have been absent at SFT6 since 1993 (see Hypothesis #1).

Source Topics: The charts below are derived from the KRIS Ten Mile Topics listed here. Check these Topics in the KRIS database to see the information in context and to access the Info Links and Metadata.

(SF) Temperature: FWA at South Fork at Camp 28 (SFT6) 1993-1998

(SF) Temperature: FWA, South Fork Sites, 1997

(SF) Fish: SF Ten Mile at Camp 28 (SFT6) Catch, 1993-2000

The chart at left from the KRIS Ten Mile database shows that the South Fork Ten Mile River warmed to levels unsuitable for coho salmon juveniles ( >16.8º C MWAT) between the upper South Fork site above Gulch 11 and the South Fork above Redwood Creek during the warmest portion of 1997.  This 2 km reach warmed 2.-2.5 degrees C for all of summer.  The upper South Fork and Gulch 11 itself are cool enough to support coho.  South Fork water temperatures continue to warm downstream to the station above Churchman Creek. Even the lowest station above Smith Creek was too warm for coho in 1997. 

The floating weekly average water temperature of the South Fork Ten Mile River below Redwood Creek was above 16.8º C in all years between 1993 and 1998 except 1994.  High water temperature is sometimes limiting the production of the coho salmon in this reach. Data from more recent years indicate that 2000 and 2001 water temperatures were below 16.8º C MWAT, as the duration and magnitude of exceedence varies between years.

 The chart at left shows that coho salmon juveniles were absent from the reach adjacent to temperature station SFT6 according to HTC electrofishing surveys from 1993 to 2000. This reach of the South Fork Ten Mile River formerly harbored coho according to CDFG (1961). Water temperature could be the factor limiting coho salmon production in at this site in some years.


From KRIS Ten Mile Maps

This image shows the location of HTC temperature data collection stations with coding as to whether temperatures in the years measured exceeded a maximum weekly floating average temperature of 16.8° C , which is the threshold for coho salmon suitability. Sites over 16.8° C MWAT in all years  are colored red, in some years yellow or in no years blue. Note that the locations of SFT11 and SFT12 are cool enough to support coho in all years but that less than 2.0 km downstream at SFT6 is not. Site SFT5 is approximately 5 km below site SFT6 and it too becomes too warm for coho in most years.

This image shows the riparian forest of the South Fork Ten Mile River from Gulch 11 to the SFT5 site above Churchman Creek, as well as lower Redwood Creek. The U.S. Forest Service vegetation and timber stand analysis based on 1994 Landsat imagery was clipped to 90 meters on each side of the stream. The SF Ten Mile riparian zone downstream of SFT11 and SFT12 has a substantial component of the trees that are early seral with Saplings (yellow = trees 1"-4.9") and small trees (olive green = 5"-11.9") predominating, but with some patches of medium large (20-29.9" diameter) trees interspersed. Small diameter trees are associated with more open airflow and this condition is promoting warming of the stream in this reach. Even reaches with larger trees showing on one side may have smaller diameter trees on the opposite stream bank indicating a disparity in tree retention in previous timber harvests and an opportunity for stream warming. Temperature monitoring sites are shown with appropriate color codes showing that Gulch 11 and the SF above are suitable for coho in all years.

This image shows a tiled aerial mosaic of 1999 photo of the South Fork Ten Mile River zoomed in to the area between Gulch 11 and Redwood Creek. Note the absence of large, mid to late seral trees, which would help to maintain shade and a cool microclimate in this reach. Hydrography is USGS 1:24,000. Blue temperature site indicators show that Gulch 11 and the South Fork above are cool enough for coho in all years (<16.8° C MWAT) but that the South Fork less than two miles downstream is too warm, indicating that the stream warmed in this reach due to local influences. The image was imported from the KRIS Mendocino Map ArcView project.

Alternative Hypotheses

Retention of direct shade, including hardwoods, prevents warming and increase in temperature is a result of other processes. 

Support for this alternative hypothesis was presented by Ambrose et al. (1996) (see quote). See also McCauley, J. and A. Goos (2000), Annotated Bibliography of Information and Studies Relevant to Temperature, Salmonids, and Water Quality assembled by the Oregon Forest Industry Council and the Washington Forest Protection Association for support of this alternative hypothesis above.

Other factors such as increased stream width and decreased depth are a more significant mechanism for stream warming.

Warming may be caused by channel width, orientation and bedrock type. Ambrose and Hines (1997) opined that high water temperature of the South Fork above Churchman Creek (SFT6) "is probably a function of its incised bedrock channel location. Beschta et al. (1987) found bedrock channels more efficient than gravel-bed channels at conducting heat." 

Monitoring Trends to Support or Refute the Hypothesis

Place temperature probes across riparian zones with different buffer retention and in the stream above and below reaches with closed and open canopy, but identical shade components, then study the relationship of riparian over-story condition, air temperature (microclimate) and water temperature collecting Ten Mile River specific data.

Monitor water temperature, and width to depth ratios in reaches upstream at various stations to determine the effect of this parameter in Ten Mile River tributaries, choosing reaches with similar canopy and riparian stand conditions to control for shade and microclimatic effects. 


Ambrose, J., D. Hines, D. Hines, D. Lundby, and J. Drew. 1996. Ten Mile River Watershed 1995 Instream Monitoring Results. Volume I. Georgia-Pacific West Inc. Fort Bragg, CA. 84 pp.

Ambrose, J., and D. Hines. 1997. Ten Mile River Watershed 1996 Instream Monitoring Results. Georgia-Pacific West Inc. Fort Bragg, CA. 55 pp.

Ambrose, J., and D. Hines. 1998. Ten Mile River Watershed 1997 Instream Monitoring Results. Georgia-Pacific West Inc. dba The Timber Company. Fort Bragg, CA. 51 pp.

Armour, C.L. 1991. Guidance for evaluating and recommending temperature regimes to protect fish. U.S. Fish and Wildlife Service. Fort Collins. Biological Report 90(22). 13 p.

Bartholow, J.M. 1989 . Stream temperature investigations: field and analytic methods. Instream flow information paper no. 13. Biological Report 89(17). U.S. Fish and Wildlife Service, Fort Collins, Co.

Brosofske, K.D., J. Chen, R. J. Naiman and J. Franklin. 1997. Harvesting Effects on Microclimatic Gradients from Small Streams to Uplands in Western Washington. Ecological Applications. 7(4): 1188-1200.  

California Department of Fish and Game (CDFG). 1961. South Fork Ten Mile River (Ten Mile River tributary) stream survey, September 1961. CDFG unpublished file memo by S.N. Nye. Yountville, CA. 5 pp.

McCauley, J. and A. Goos. 2000. Annotated Bibliography of Information and Studies Relevant to Temperature, Salmonids, and Water Quality. Oregon Forest Industries Council and Washington Forest Protection Association. 7 p.

Poole, G.C., and C.H. Berman. 2000. Pathways of Human Influence on Water Temperature Dynamics in Stream Channels. U.S. Environmental Protection Agency, Region 10. Seattle, WA. 20 p.

Schwind, B. 1999. USFS Thematic Mapper Landsat Vegetation Coverage Background Information. USDA Forest Service. Pacific Southwest Region Remote Sensing Lab. Sacramento, CA.

Warbington, R., B. Schwind, C. Curlis and S. Daniel. 1998. Creating a Consistent and Standardized Vegetation Database for Northwest Forest Plan Monitoring in California. USDA Forest Service. Pacific Southwest Region Remote Sensing Lab. Sacramento, CA.

Welsh, H.H., Jr., G.R. Hodgson, B.C. Harvey, and M.F. Roche. 2001. Distribution of juvenile coho salmon in relation to water temperatures in tributaries of the Mattole River, California. North American Journal of Fisheries Management 21(3):464-470. [335k]