All of the SOD rates measured by USGS personnel in the Tualatin River Basin from 1992 through 1996 are shown both in table 1 and figure 5. Ninety-seven rates were measured at nine main-stem sites from 1992 through 1994. Twenty-eight rates were measured at 11 tributary sites from 1995 through 1996. All of the measurements were made during the May through October low-flow period, with a slight emphasis on the month of September, which is a critical period for DO in the main-stem river. During each measurement, the amount of oxygen in the chamber was not a limiting factor. The lowest initial and final DO concentrations encountered were 4.9 and 3.9 mg/L, respectively. Most of the measurements, however, were performed with DO concentrations well above 6.0 mg/L.
Table 1. Rates of sediment oxygen demand measured in the main stem and selected tributaries of the Tualatin River, Oregon, by U.S. Geological Survey personnel, 1992 through 1996
[SOD T, rate of sediment oxygen demand measured at river water temperature; SOD 20 , rate of sediment oxygen demand corrected to 20 degrees Celsius (°C) using SOD 20 = SOD T /1.065 T-20 where T is water temperature in °C; g/m 2 d, grams of oxygen per square meter per day]
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Water
Map SODT temperature SOD20
number Site Date (g/m2d) (°C) (g/m2d)
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Main stem Tualatin River sites, by river mile (RM)
1 Tualatin River at Stafford Road near Lake 06/17/92 3.6 18.4 4.0
Oswego, Oregon (RM 5.5) 2.2 18.6 2.4
10/05/92 2.2 16.1 2.9
2.9 16.1 3.7
05/24/93 2.8 17.2 3.4
3.2 17.5 3.8
2.2 17.2 2.7
2.8 17.5 3.3
2.9 17.2 3.4
3.7 17.5 4.4
09/09/94 2.8 19.2 2.9
2.3 19.2 2.4
2.7 19.2 2.8
2.5 19.2 2.7
2.9 19.2 3.0
2 Tualatin River at Boones Ferry Road at 06/15/92 1.9 18.9 2.0
Tualatin, Oregon (RM 8.7) 2.6 19.2 2.7
08/17/92 2.0 20.8 1.9
10/07/92 .5 15.1 .6
.5 14.8 .6
3 Tualatin River at Cook Park near Tigard, 05/15/92 1.2 15.9 1.6
Oregon (RM 10.0) 2.1 16.4 2.6
4 Tualatin River near Highway 99W Bridge 06/16/92 1.9 18.3 2.1
near King City, Oregon (RM 11.7) 2.0 18.3 2.2
08/20/92 2.1 21.1 2.0
1.8 21.0 1.7
05/25/93 1.5 16.8 1.8
1.9 16.8 2.3
08/30/93 2.4 18.7 2.6
1.6 18.7 1.7
1.4 18.7 1.5
1.9 18.4 2.1
1.9 18.5 2.1
09/08/94 3.8 18.8 4.1
3.4 18.8 3.7
2.2 18.8 2.4
2.3 18.8 2.5
3.0 18.8 3.2
5 Tualatin River at Elsner Road near 06/18/92 1.8 16.5 2.2
Sherwood, Oregon (RM 16.2) 1.9 16.6 2.8
08/18/92 3.2 21.2 3.0
2.6 21.4 2.4
10/06/92 1.2 14.8 1.7
1.3 14.8 1.7
6 Tualatin River at Lakeside Reclamation 08/19/92 1.8 20.9 1.7
near Scholls, Oregon (RM 20.3) 1.6 20.9 1.5
10/08/92 2.3 14.4 3.2
1.5 14.4 2.1
05/26/93 1.4 16.0 1.8
1.2 16.0 1.5
2.2 16.1 2.8
1.1 16.4 1.4
1.8 16.5 2.2
1.8 16.4 2.2
09/03/93 1.5 18.4 1.7
2.9 18.3 3.2
2.1 18.3 2.3
09/07/94 1.6 17.7 1.8
2.3 17.7 2.6
.8 17.7 .9
1.9 17.7 2.2
2.6 17.7 3.0
2.5 17.7 2.9
7 Tualatin River at Highway 210 Bridge 05/27/93 2.2 16.5 2.7
near Scholls, Oregon (RM 26.9) 1.7 16.6 2.1
1.8 16.6 2.3
2.6 16.6 3.2
1.2 16.6 1.5
1.5 16.7 1.9
09/02/93 2.6 17.7 3.0
2.1 18.2 2.3
1.9 17.8 2.1
2.2 18.2 2.4
1.7 18.0 1.9
09/15/94 1.5 16.5 1.8
1.2 16.5 1.5
1.4 16.5 1.8
1.4 16.5 1.7
2.6 16.5 3.3
8 Tualatin River at Meriwether irrigation 08/31/93 1.7 15.8 2.2
pump near Hillsboro, Oregon (RM 36.8) 1.1 16.5 1.4
1.5 15.9 1.9
1.4 16.7 1.7
1.2 16.1 1.5
09/12/94 1.3 15.8 1.8
2.0 15.8 2.5
1.2 15.8 1.5
9 Tualatin River at river mile 43.2 near 09/01/93 1.7 15.4 2.3
Hillsboro, Oregon (Jackson Bottom) 1.8 15.3 2.4
1.6 15.8 2.0
1.1 15.1 1.4
1.9 15.7 2.4
09/14/94 2.0 15.3 2.7
2.2 15.3 2.9
1.9 15.3 2.5
3.1 15.3 4.2
1.8 15.3 2.4
Tualatin River tributary sites
10 Beaverton Creek at Arleda Park near 07/16/96 4.9 22.6 4.1
Hillsboro, Oregon 5.6 22.6 4.7
11 Bronson Creek at Walker Road at 07/18/96 12.7 22.4 10.9
Hillsboro, Oregon 4.9 22.4 4.2
8.5 22.4 7.3
12 Cedar Mill Creek near Jenkins Road at 07/19/96 5.4 18.0 6.1
Beaverton, Oregon 2.5 18.0 2.8
5.3 18.0 6.0
13 Dairy Creek at Dairy Creek Park at 08/07/95 2.7 18.0 3.1
Hillsboro, Oregon
14 Fanno Creek at Durham City Park at 07/15/96 1.0 23.0 .9
Durham, Oregon .3 23.0 .2
15 Fanno Creek at Fanno Creek Park at 08/08/95 2.7 20.0 2.7
Tigard, Oregon
16 Fanno Creek at Englewood Park at 07/15/96 3.2 22.3 2.7
Tigard, Oregon 5.1 22.3 4.4
5.0 22.3 4.3
17 Gales Creek at Zurcher irrigation pump 07/19/96 7.0 16.0 9.0
near Forest Grove, Oregon 2.2 16.0 2.8
1.9 16.0 2.5
18 Rock Creek near Southeast 59th Avenue 08/07/95 2.1 18.0 2.4
at Hillsboro, Oregon 07/18/96 1.9 17.5 2.2
2.6 17.5 3.0
5.1 17.5 6.0
19 Rock Creek at Rock Creek Wastewater 07/17/96 2.8 19.4 2.9
Treatment Plant at Hillsboro, Oregon 4.0 19.4 4.2
1.6 19.4 1.7
20 Willow Creek at Apollo Ridge Park at 07/16/96 1.7 19.6 1.7
Beaverton, Oregon 4.6 19.6 4.7
4.3 19.6 4.4
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Figure 5. Rates of sediment oxygen demand (SOD20, corrected to 20 degrees Celsius) measured in the main stem and selected tributaries of the Tualatin River, Oregon by U.S. Geological Survey personnel, 1992- 1996.
The data presented in table 1 and figure 5 were not corrected for water-column oxygen demand, as discussed in the methods section. The BOD measured in the stream water at the sites and times when SOD rates were measured was less than 3 percent of the oxygen loss observed in the SOD chambers. The BOD samples collected in conjunction with these SOD measurements confirmed that a moderate level of BOD does not consume enough oxygen over 2 hours in a volume of 52 liters of water to be important relative to an SOD on the order of 2 g/m 2 d.
The SOD rate data are highly dependent not only on the ability of the HydrolabTM to accurately measure oxygen concentration, but also on the volume of water in the chamber and the area of sediment to which that water is exposed. The HydrolabsTM used proved reliable; pre- and post-calibrations showed minimal instrument drift over a 2- to 5-hour period. Therefore, these instruments were not considered to be a significant source of error in the SOD rates. The area of exposed sediment was determined largely by the size of the chamber. The roughness of the sediment surface introduces some uncertainty into the determination of the effective sediment area, but probably results in an error of no more than a few percent. The volume of isolated water was the least well characterized variable in the calculation of the SOD rate. When the chambers were seated, it was noted whether the "ideal" insertion depth was achieved and, if not, how high or low it was seated relative to that ideal. Volume adjustments were made in the subsequent calculation of the SOD rate (equation 1) on the basis of those observations of insertion depth. Estimates of the actual insertion depth are probably accurate to within one-half inch, resulting in a maximum volume error of about 2.9 liters, or less than 6 percent. On the basis of a maximum volume error of 6 percent and an area error of no more than 3 percent, the total error in these measured SOD rates should be less than 10 percent.
Despite the fact that each individual measurement of SOD is believed to have an error of less than 10 percent, the data in figure 5 indicate that the variability of this rate at any one stream site is much higher than the analytical uncertainty. Coefficients of variation for the rates measured at main-stem sites range from 19 to 31 percent. The variation is probably due to the heterogeneous nature of the bottom sediments; each of the individual measurements was obtained with a different 0.225 m 2 of sediment surface. Although the bottom of the Tualatin River from RM 5.5 to 43.2 is predominantly silty and contains a large amount of detrital material and woody debris, simple visual inspection revealed a large amount of variation. Some sedimentary pockets of very fine-grained, almost gelatinous material (the divers described it as "pudding") were observed at most sites. Some of the sediments were characterized by so much silt that the divers had to be careful not to insert the chambers too deeply; other areas were harder and contained more clay. Each site had some of each of these types of sediment.
Overall, the measured SOD rates range from 0.2 to 10.9 g/m 2 d with a median of 2.4 g/m 2 d. Fifty percent of the measurements (the interquartile range) fall between 1.8 and 3.0 g/m 2 d. These values, especially those from the main-stem Tualatin River sites, are similar to USGS measurements of SOD in 1994 at 15 sites along the lower 50 miles of the Willamette River (1.3 to 4.1 with a median of 2.0 g/m 2 d, Caldwell and Doyle, 1995). This similarity is not surprising; the sediments of the two rivers are both silty with a moderate content of organic matter (visual determination). Indeed, the range of rates obtained in this study is similar to that obtained for numerous streams with silty sediments elsewhere, excluding those that are heavily impacted by pollution (Murphy and Hicks, 1986; ENSR R&D, 1994; Peter Nolan, U.S. Environmental Protection Agency, personal commun., 1997).
In other waters, the ODEQ found moderate levels of SOD (2.4 g/m 2 d) in Rickreall Creek near Dallas, Oregon (Larry Caton, ODEQ, personal commun., 1997). ENSR R &D (1994) measured rates of 0.63 g/m 2 d in the Chehalis River (Washington), 1.02 g/m 2 d in Lake Washington (Seattle, Washington), and roughly 1.9 g/m 2 d in the Columbia Slough (Portland, Oregon). Butts (1974) measured rates ranging from 0.56 to 5.0 g/m 2 d in the Upper Illinois Waterway. Murphy and Hicks (1986) documented SOD rates at many sites in the southeastern United States and found a range of from 0.5 to 16.6 g/m 2 d. (All of these SOD rates were corrected to 20°C.) Typically, the lowest rates (0.5--1 g/m 2 d) are observed for sediments with a high sand component and little organic matter, moderate rates (1--3 g/m 2 d) are found for silty sediments containing a moderate amount of organic matter, and the highest rates are found at sites polluted with organic sludge from various point sources (Peter Nolan, U.S. Environmental Protection Agency, personal commun., 1997).
The SOD rates measured in the Tualatin Basin are large enough to be important sinks for dissolved oxygen. In fact, when the BOD is low or the streamis shallow, an SOD of 2 g/m 2 d potentially can be the largest sink for dissolved oxygen in the stream. For example, an SOD rate of 2 g/m 2 d exerted in a stream with a 3-foot depth, a depth similar to that found in many of the Tualatin's tributaries, will consume more than 2 mg/L of dissolved oxygen in 1 day. For smaller, shallower tributaries, the impact can be even greater. In the reservoir reach of the Tualatin River, where the average depth is typically 10 to 15 feet, an SOD of2 g/m 2 d will consume 0.4 to 0.7 mg/L from the average water-column DO concentration in one day. Because the residence time in this reach is long (up to 14 days), the impact of the SOD can be considerable.
Beyond the intrinsic variability of the SOD rate at any given site, the data suggest that the rates observed at the tributary sites are higher than the rates observed at the main-stem sites (fig. 6). Figure 6 is a box and whisker plot. In this type of plot, the whiskers extend to the 10th and 90th percentiles of the data, the box extends from the 25th to the 75th percentile, and the median is represented by a horizontal line within the box. Data that fall beyond the range of the whiskers are plotted as individual circles. The median SOD rate for the main stem (n=97) is 2.3 g/m 2 d, while the median rate for the tributary sites (n=28) is 3.6 g/m 2 d. A one-factor ANOVA on the original and the rank-transformed data showed that the measured rates from these two groups of sites have statistically different medians at the 95 percent confidence level. A Mann-Whitney test also showed that these two groups were not drawn from the same population.
Figure 6. Sediment oxygen demand as a function of stream order. (Main-stem sites include 9 sites from river mile 5.5 to 43.2; tributary sites include 11 sites on 8 tributaries.)
The reason for this difference is less apparent. Two different hypotheses seem worth further investigation. First, all of the main-stem rates were measured during the 1992 to 1994 period, and most of the tributary rates were measured during 1996 after a major flood in February 1996. The flood might have washed a great deal of fresh, labile organic material from the land into the tributaries (and main stem), causing an inflated SOD rate that will decrease over the coming years as that organic matter ages or is washed farther downstream. The second hypothesis is that the tributaries normally exhibit higher rates of SOD due to their closer interaction with the landscape and their closer proximity to sources of fresh, relatively labile organic matter.
This report offers no data to support or refute either of these two hypotheses. Further investigations of SOD rates in both the main stem and the tributaries will help to resolve this issue. However, the fact that (a) the main stem and tributaries are subject to some amount of flooding during each and every winter high-flow season, and (b) particulate organic material that deposits in the main stem probably has aged and become less labile while in transit to that location lends credence to the second hypothesis--the SOD rates exerted in the tributaries normally are higher than those in the main stem.
Within the main stem of the river, the SOD rate does not seem to vary much with location (fig. 5). Measurements were taken in both the meander reach (RM 55.3 to 33.3), and in the reservoir reach (RM 33.3 to 3.4). Both reaches flow slowly enough in the summer to trap sediment, but the reservoir reach in particular is a sedimentary zone that traps a large amount of detrital organic matter. One might expect that the reservoir reach would accumulate more detrital material and therefore exert a higher SOD rate as that material decomposed.
An analysis of the effects of river flow regime (meander reach versus reservoir reach) on the SOD rate must account for other sources of variation, such as any within-site and site-to-site differences. To account for and separate these effects, a nested ANOVA is necessary (Box and others, 1978). This data set does not have an identical number of measurements at each site nor the same number of sites in each flow regime; therefore, this is an unbalanced, nested design. The data were analyzed only for those sites where more than five measured rates were obtained; RMs 8.7 (n=5) and 10.0 (n=2) were left out. Performing an unbalanced, nested ANOVA (SAS Institute Inc., 1989) on the remaining main-stem SOD data produced similar results for both the rank-transformed and the original data. Using Satterthwaite's approximate test procedure on the ANOVA results, no significant differences in the rate data were found due to the two flow regimes (fig. 7).
Figure 7. Variation in sediment oxygen demand among main-stem sites due to differences in river flow regime. (Reservoir sites include river miles 5.5, 11.7, 16.2, 20.3, and 26.9; meander sites include river miles 36.8 and 43.2.)
By far, most of the differences in the main-stem rate data were due to within-site variability, which accounted for roughly 70 percent of the total variance. Site-to-site differences accounted for the remaining 30 percent of the variance. Tukey's multiple comparison test, using either the original or the rank-transformed data, showed that the SOD rates measured at RM 5.5 (Stafford Road) were significantly higher than those measured at most of the other main-stem sites. The measured rates from the other sites (n> 5), however, were not significantly different from one another. These higher rates at RM 5.5 will be discussed in more detail in the next section.
During the low-flow season, the residence time of a water parcel in the reservoir reach of the Tualatin River can be as long as 2 weeks. In conjunction with warm water temperatures, ample solar insolation, and a sufficient nutrient supply, the residence time in this reach is sufficient to produce large blooms (> 50 µg/L chlorophyll- a ) of phytoplankton. The phytoplankton population generally starts to become significant near the start of the reservoir reach (RM 33.3) and attains fairly high levels by the middle of that reach (about RM 18). The reach from RM 18 to 30 is a transition zone from a " low-concentration" chlorophyll- a region upstream of the reservoir reach to a " high-concentration" chlorophyll- a reach in the lower part of the reservoir. Figure 8 illustrates this trend in the measured chlorophyll- a data for the period over which the main-stem SOD data were collected (May through October 1992-94).
Figure 8. Measurements of chlorophyll- a at main-stem sites on the Tualatin River during the period May through October of 1992-94. (Data from Unified Sewerage Agency of Washington County; available in STORET.)
These algal populations produce a large amount of biomass, some of which settles to the bottom of the river and decomposes along with other detrital material. It has been hypothesized that this additional algal detritus, which presumably is relatively labile, would increase the rate of SOD at sites in the "high chlorophyll" zone of the reservoir reach. The rate data collected in this study can be used to test this hypothesis in two ways: spatially and temporally.
With regard to spatial differences, if the phytoplankton die and settle to the river bottom in much the same spatial pattern as the algal population (fig. 8) and if algal detritus increases the SOD rate, then one would expect to find higher SOD rates at sites in the high chlorophyll zone. This hypothesis was tested by grouping the SOD sites according to the three zones in figure 8 and performing a nested ANOVA. Ignoring sites with five or fewer data points, this design groups sites from RMs 5.5, 11.7, and 16.2 in the high chlorophyll zone, RMs 20.3 and 26.9 in the transition zone, and RMs 36.8 and 43.2 in the low chlorophyll zone (fig. 9).
Figure 9. Variation in sediment oxygen demand among main-stem sites due to the size of the phytoplankton population. (Sites with high chlorophyll- a concentrations include river miles 5.5, 11.7, and 16.2; sites in the chlorophyll- a transition zone include river miles 20.3 and 26.9; sites with low chlorophyll- a concentrations include river miles 36.8 and 43.2.)
An unbalanced, nested ANOVA was performed on the rank-transformed chlorophyll- a data to determine whether the apparent trend in chlorophyll- a concentration from one zone to another in figure 8 was statistically significant. Indeed, a significant difference was found among the zones, and Tukey's multiple comparison test resulted in site groups for the three chlorophyll zones that were almost identical to those in figure 8. In contrast, an unbalanced, nested ANOVA performed on either the rank-transformed or the original SOD rate data showed no significant differences between the SOD rates measured in different chlorophyll zones. Instead, the analysis showed that most (70 percent) of the SOD variability was due to within-site heterogeneity and the balance was due to other site-to-site differences.
One site in the high chlorophyll zone, RM 5.5, does have a median SOD rate (3.0 g/m 2 d) that is significantly higher, by roughly 36 percent, than the median rate for all other main-stem sites (2.2 g/m 2 d) and all other sites in the high chlorophyll zone (2.2 g/m 2 d, n> 5). A one-factor ANOVA and Tukey's multiple comparison test performed on the chlorophyll data only within the high chlorophyll zone shows that the median chlorophyll- a concentration at RM 5.5 is significantly higher than at other sites in the high chlorophyll zone where SOD rates were measured (RMs 8.7, 11.7, and 16.2). This result suggests that decomposing algal detritus may contribute to the increased SOD rate at RM 5.5.
Other factors, however, might also be partly responsible for the increased SOD rate at RM 5.5. The bathymetric conditions, for example, might be an important factor. The river is deep (18 feet) at that site, compared with many of the other SOD sites, and is bounded on both the upstream and downstream sides by sills that tend to make this site act as a more efficient depositional area, trapping more detritus than other sites in this reach. Therefore, the bathymetric characteristics of this site may make it unrepresentative of the lowest part of the reservoir reach.
The influence of phytoplankton on the SOD rate can be tested temporally as well as spatially. If the deposition of fresh algal detritus had an effect on the SOD rate, then one would expect the measured SOD rate at a particular site in August or September to be significantly higher than that found at the same site in May or June, before any algal detritus from that season's blooms had been deposited. Four sites on the main stem Tualatin River were used to investigate this potential seasonal dependence. The measured SOD rates at RMs 5.5, 11.7, 20.3, and 26.9 are shown in figure 10, separated according to the time of year. Measurements in May and June were grouped together to represent conditions before any significant algal blooms; generally the first blooms of the year do not occur until mid-June or July, after river discharge decreases to less than 300 ft 3 /s. Measurements in August, September, and October were grouped to represent conditions after the deposition of fresh algal detritus.
Figure 10. Seasonal variations in sediment oxygen demand at four main-stem sites. ("Early" indicates measurements made in May and June; "late" indicates measurements made in August, September, and October.)
Separate rank-transformed ANOVA and Mann-Whitney tests for each of these four sites revealed no significant seasonal differences in the measured SOD rates at any of these sites (RM 5.5, p=0.12; RM 11.7, p=0.73; RM 20.3, p=0.45; RM 26.9, p=0.80). Any small effect of the deposition of fresh algal detritus, if present, is masked by the intrinsic heterogeneity of the SOD rate at each site.
This result, and that from the spatial analysis, indicate that algal detritus is not a primary source of organic matter for the SOD. The decomposition of algal detritus may contribute to the SOD at some locations, but most of the SOD is likely to originate from nonalgal sources of organic matter.
Sediment Oxygen Demand in the Tualatin River Basin, Oregon, 1992-96
U.S. Geological Survey