Land Use Effects on the Water Quality of Mill Creek
by Suzy Oversvee & Sarah Pulliam

The headwaters of Mill Creek are located in the Blue Mountains, Umatilla County, Oregon.  The river flows north from the mountains and then west through the City of Walla Walla, Walla Walla County, Washington, providing the city with drinking and irrigation water. The headwaters of Mill Creek are in a portion of the Umatilla national Forest set aside as a watershed for the City of Walla Walla.  This watershed has a grass-tree mosaic vegetation pattern, with forests limited to shady north-facing slopes (see vegetation for more information).

The Mill Creek watershed is home to elk, cougar, deer, and bear.  Bird species include grouse, hawks, and songbirds (see wildlife) Mill Creek provides habitat for resident trout and other small fish (see fish).

Human settlement dates back at least a century. Mill Creek has been heavily altered due to the human impact on the surrounding environment.  The human influence on the Mill Creek drainage basin is predominantly due to agriculture, grazing, forestry, and urbanization.  Homes have been built along the banks of Mill Creek, most densely concentrated near Kooskooskie, Washington.  This project aims to determine some effects that the greater Walla Walla community has on Mill Creek and to see how different land uses each impact water quality of Mill Creek.

Where the river runs into Walla Walla, it is diverted by a levee and channeled by concrete corridors through the city, in some instances underneath roadways, in other cases under direct sun with no riparian vegetation. We established six sample sites along Mill Creek, from near the headwaters to the town of College Place, which is east of the water treatment plant serving the City of Walla Walla:

1. 25’ upstream of the bridge that allows traffic from Mill Creek Road to cross Mill Creek into Tiger Canyon (Umatilla County).

Near Tiger Canyon (above)

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2. Johnston Wilderness Campus, Whitman College. 5 ft. upstream of the access bridge from Mill Creek Rd. to the Campus (Umatilla County).

Next to the Johnston Wilderness Campus (above)

3. At the old Walla Walla water intake, 100 ft. below the concrete weir (Walla Walla County).

The old Walla Walla water intake (above)

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4. Five Mile Rd., under the bridge that passes over Mill Creek (Walla Walla County).

Near Five Mile Road (above)

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5. Rooks Park, below the diversion dam, under the footbridge spanning Mill Creek in the park (Walla Walla County).

Rooks Park (above)

6. College Place, under the College Avenue Bridge that spans Mill Creek. (Walla Walla County).

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At each sampling site we performed nitrate (NO3-), phosphorous (PO43-), and nitrite (NO2-) tests using a Hach water analysis kit (Hach Co., Loveland, Colorado).  We followed the instructions provided by the Hach Company for use of the kit.  Dissolved oxygen and temperature were determined with a YSI model 58 dissolved oxygen meter (Yellow Springs, OH) and pH was taken with a standard litmus test.  Instructions for use of the dissolved oxygen meter were developed and provided by Dr. Charles Drabek of the Whitman College Biology Department.

Samples were made both before and after winter rains.  The “dry” samples taken from sites 1-3 were taken on October 23, 1999, and sites 4-6 were sampled on October 24, 1999.  The “wet” samples from sites 1-5 were taken on November 12, and site 6 was sampled on November 18.

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Why we Chose These Tests

pH was measured to determine how acidic or basic the stream is.  Water that is either too acidic or too basic has the potential to severely alter the ecosystem of the stream.  In particular, game fish, amphibians, and some aquatic insects are especially sensitive to high levels of acidity.

Temperature was measured because it is a good indicator of how well the riparian vegetation is providing shade for the creek.  Generally, the less riparian vegetation present, the higher the temperature will be which could alter the creek’s ecosystem.  For example, the upper temperature limit for salmonoid survival is 70 degrees Fahrenheit.  If the temperature were to get warmer, salmonoids would not survive.

Dissolved oxygen indicates the level of pollution in a stream.  In polluted water, there are generally more decomposers present that consume the dissolved oxygen as opposed to producers (plants) that produce oxygen.  A lack of riparian vegetation could also contribute to a decrease in dissolved oxygen, which is needed for healthy organisms within the stream.

High phosphate concentrations stimulate algal growth.  If too much algae are present in the stream, a process called eutrophication takes place, where rampant growth of algae causes an initial increase of dissolved oxygen in the water.  When algae dies, decomposer activity depletes the oxygen supply in the water because the process of decay is oxygen intensive.  Thus, when eutrophication takes place, it is difficult for the stream to support animal life.  Phosphates are found in detergents and inorganic fertilizers, so they can easily find their way to waterways in urban and agricultural areas.

Nitrites and nitrates often come from dead organisms and urine and feces.  Nitrates specifically are found in fertilizers.  High concentrations of nitrites and nitrates can therefore be attributed to agriculture due to runoff from the fields and from cattle grazing in or near streams.

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What we found

pH:  The pH level at each sample site ranged between 5 and 6 in October and between 5.5 and 6 in November, increasing downstream toward College Place.  There was not a large difference between the two trials.

Temperature:  In October, the temperature of the stream increased gradually going downstream.  In November, the temperature increased from Tiger Canyon to Five Mile Creek, but then showed a dramatic decrease at Rooks Park and then regained the previous trend to College Place.  The data taken for the second trial was split into two days.  The first day the outdoor temperature was between 10 and 20 degrees warmer than on the second day data was taken, which may explain this break in data (Figure 1).

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Dissolved oxygen:  Our data shows a steady decrease in dissolved oxygen going
downstream in October, whereas there does not seem to be a predominant trend in the data collected in November.  The volume of water in the creek appeared to increase after the rains came, which may have had an effect on the data collected then (Figure 2).

Phosphate concentration:  The trends between the data from the two trials are fairly well matched with a large increase at College Place (Figure 3).  We believe that this increase is due to the fact that Mill Creek is channeled through the City of Walla Walla and receives effluent water containing soap and other detergents.

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Nitrite:  For both trials, the nitrite concentrations are very similar, gradually increasing
downstream with the exception that the November trial shows a dramatic increase at
College Place.  This increase indicates that the source of the increase comes from Walla
Walla, the urbanized segment of the stream.  Increased runoff from the previous rains may have contributed to these results (Figure 4).

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Nitrate:  The trends in data between the two trials are similar, with the exception of the
outlier for the October trial at the Johnston Wilderness Campus.  Again, there is a fairly
large increase in the concentration at College Place (Figure 5).

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Based on our data, especially our phosphate, nitrite, and nitrate concentration data, urbanization appears to have the most impact on Mill Creek as it is channeled through Walla Walla.  Except for the nitrate tests, the second trial after the rain generally did not produce dramatically different results.  This may be due to the fact that we did not start the second trial until 3-4 days after there was rain, so the effects of increased runoff may not have been as acute as if we had run the tests during or immediately after there had been rain.

Further studies should be conducted to gain more extensive knowledge on this matter.  The data that we obtained, though, indicate that, in order to reduce our negative impact on Mill Creek, focus should be put on Walla Walla where Mill Creek is channeled through town.

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