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Water Temperature


Water temperature is one of the more critical parameters affecting fish migration behavior during the April through September adult and juvenile salmonid migration periods. The optimal temperature range during the summer juvenile and adult migration period is generally recognized to be between 10 to 20°C (45 to 68°F) (Bonneville Power Administration, BPA 1995). The upper tolerance limit is considered to be 21°C (70°F), depending on whether fish have had sufficient time to acclimate to increasing temperatures. Water temperatures above 21°C (70°F) can have lethal effects on salmonid fishes if these high temperature waters cannot be avoided (BPA 1995). However, salmon stocks have adapted certain life stages to higher seasonal temperatures in the southern part of their range.
Historic water summer temperatures in the Snake River basin far exceeded the optimal ranges mentioned above. Adaptations included spring and summer chinook migrating into higher elevation tributaries to spawn so their young could rear where water temperatures were cooler. Snake River coho, sockeye, and steelhead adapted similar to the spring/summer chinook. Fall chinook spawned in the mainstem (usually near the mouth of major tributaries), about 95% of them upstream from the Lower Snake River. Their life history was likely adjusted in avoidance of hot summer water temperatures in the Lower Snake River by migrating before the heat of the summer when shoreline rearing areas heated up. Juvenile fall chinook from above Hells Canyon probably reached the Lower Snake River before peak hot temperatures in the summer. Juvenile fall chinook from Hells Canyon, the Lower Clearwater River, and the Lower Snake River probably moved through the Lower Snake River to rear in the slightly cooler waters of the lower Columbia River (now McNary, John Day, The Dalles, and Bonneville Reservoirs) if they had not experienced a sufficient growth period in the middle or upper Snake River.
Water temperatures in the Lower Snake River are relatively cool in May and June during the peak flow and snowmelt period, with typical readings ranging from 10 to 14°C (50 to 57°F). By mid- to-late July, however, temperatures usually warm up to 22°C to 24°C (71.6 to 75.2°F) and remain above 20°C (68°F) until late September. The highest temperatures generally occur from August to mid-September (BPA 1995). The late-summer maximum temperatures suggest that the most significant effect of hydropower dam construction may be that the period of maximum temperatures has shifted from mid-July through August to mid-August through September (EPA and NMFS 1971; BPA 1995). This is based upon a comparison with temperature data collected prior to dam construction (1955-1958) in the Lower Snake River, where maximum temperatures were frequently above 22°C (72°F) from mid-July to late August (FWPCA 1967). Similarly surface water temperature data (1 m depth) collected at SNR-107, prior to construction of the Lower Granite Dam, reached peak temperatures in excess of 22° C between mid-July and late-August.
Since each of the Corps dams became operational, the Corps has recorded daily water temperatures passing through the dams and reported that information with adult fish count information. The fishery agencies and Corps agreed years ago that the scroll case water temperatures would be the best representation of the average water temperature the fish would experience. The scroll case draws water from all depths of the reservoir, and passes that water over the turbine blades to drive the generators of the dam.
Maximum scroll case temperatures are represented in Table 13 for Ice Harbor, Lower Monumental, Little Goose, and Lower Granite dam. There is a break in the data for Little Goose Dam from 1982 through 1990 when adult fish were not counted. Also shown in the table is the period of time that the water temperature exceeded the state temperature standard (20°C or 68°F).
Table 13. Maximum water temperatures at Corps dams (Corps 1999)

Year

Ice Harbor

Lower Monumental

Degrees F

Degrees C

Days
Over
68


First Day

Last Day

Degrees F

Degrees C

Days
Over
68


First Day

Last Day

62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98

76
76
72
75
75
76
75
73
73
74
73
72
72
71
71
73
72
73
72
73
72
73
73
75
75
72
72
71
73
74
71
68
70
70
70
71
73

24.44
24.44
22.22
23.89
23.89
24.44
23.89
22.78
22.78
23.33
22.78
22.22
22.22
21.67
21.67
22.78
22.22
22.78
22.22
22.78
22.22
22.78
22.78
23.89
23.89
22.22
22.22
21.67
22.78
23.33
21.67
20.00
21.11
21.11
21.11
21.67
22.78

60
71
47
42
60
75
54
57
61
54
36
42
46
29
44
43
28
74
48
55
35
40
60
51
73
81
53
50
70
49
43
0
18
18
41
44
52

16 July
13 July
15 July
21 July
14 July
12 July
9 July
19 July
13 July
23 July
9 Aug
22 July
30 July
28 July
30 July
27 July
3 Aug
19 July
31 July
29 July
14 Aug
8 Aug
20 July
17 July
9 July
4 July
27 July
25 July
24 July
1 Aug
16 July
--
16 July
25 July
23 July
21 July
*19 July

19 Sep
21 Sep
30 Aug
31 Aug
11 Sep
30 Sep
9 Sep
13 Sep
11 Sep
16 Sep
13 Sep
7 Sep
13 Sep
31 Aug
16 Sep
7 Sep
8 Sep
30 Sep
16 Sep
30 Sep
17 Sep
16 Sep
17 Sep
5 Sep
19 Sep
22 Sep
17 Sep
12 Sep
1 Oct
18 Sep
10 Sep
--
5 Aug
11 Aug
1 Sep
5 Sep
*8 Oct

74
75
73
72
71
70
70
71
72
73
71
74
72
74
73
73
74
71
72
71
73
74
71
68
71
70
70
71
73

23.33
23.89
22.78
22.22
21.67
21.11
21.11
21.67
22.22
22.78
21.67
23.33
22.22
23.33
22.78
22.78
23.33
21.67
22.22
21.67
22.78
23.33
21.67
20.00
21.67
21.11
21.11
21.67
22.78

53
54
39
43
48
33
41
35
38
67
40
55
52
42
49
54
52
71
50
49
59
44
50
0
30
23
41
28
75

10 July
22 July
5 Aug
25 July
27 July
31 July
7 Aug
27 July
30 July
24 July
24 July
1 Aug
26 July
5 Aug
26 July
10 July
9 July
12 July
25 July
25 July
30 July
5 Aug
10 July
--
*13 July
19 July
20 July
3 Aug
17 July

3 Sep
13 Sep
13 Sep
5 Sep
12 Sep
1 Sep
7 Sep
11 Sep
5 Sep
28 Sep
2 Sep
24 Sep
15 Sep
17 Sep
12 Sep
1 Sep
20 Sep
20 Sep
12 Sep
11 Sep
26 Sep
17 Sep
13 Sep
--
*20 Sep
10 Aug
29 Aug
8 Sep
30 Sep



Year

Little Goose

Lower Granite

Degrees F

Degrees C

Days
Over
68


First Day

Last Day

Degrees F

Degrees C

Days
Over
68


First Day

Last Day

62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98

76
73


74
74
70
71
72
72
74
73
73
73
76
72
72
72
72
71
71
72

24.44
22.28


23.33
23.33
21.11
21.67
22.22
22.22
23.33
22.78
22.78
22.78
24.44
22.22
22.22
22.22
22.22
21.67
21.67
22.22

54
42


46
51
37
38
26
29
64
43
61
49
No data
No data
No data
No data
No data
No data
No data
No data
55
49
40
28
26
53
57
82

18 July
1 Aug


13 July
23 July
25 July
28 July
10 Aug
30 July
22 July
22 July
23 July
29 July
--
--
--
--
--
--
--
--
23 July
4 July
8 Aug
*8 July
16 July
12 July
1 Sep
12 July

9 Sep
12 Sep


2 Sep
14 Sep
30 Aug
13 Sep
4 Sep
27 Aug
24 Sep
3 Sep
21 Sep
15 Sep
--
--
--
--
--
--
--
--
16 Sep
10 Sep
29 Sep
*2 Oct
9 Aug
2 Sep
26 Sep
1 Oct

76
72


76
75
74
74
78
74
74
74
74
74
73
73
74
77
76
72
69
73
68
70
71
70

24.44
22.22


24.44
23.89
23.33
23.33
25.56
23.33
23.33
23.33
23.33
23.33
22.78
22.78
23.33
25.00
24.44
22.22
20.56
22.78
20.00
21.11
21.67
21.11

35
51


49
35
59
39
64
46
41
46
49
62
74
85
47
77
55
25
8
32
0
23
26
36

21 July
18 July


28 July
20 July
17 July
21 July
17 July
26 July
30 July
23 July
7 July
30 June
26 June
25 June
13 July
3 July
12 July
*1 July
18 Aug
*17 July
--
22 July
21 Aug
*10 July

25 Aug
10 Sep


5 Sep
10 Sep
17 Sep
28 Aug
18 Sep
12 Sep
9 Sep
6 Sep
28 Aug
12 Sep
15 Sep
20 Sep
28 Aug
18 Sep
12 Sep
*28 Aug
5 Sep
*11 Sep
--
16 Aug
17 Sep
*25 Sep

NOTES: Highest temperatures usually occur in August at all dams, but with unseasonably warm weather, may occur in late July or with prolonged hot weather, in September. Blanks for Little Goose (1983-90) are for years when data was not reported.
*Temperatures over 68 degrees F occurred between 2 periods:

LM, 1994
13 July-21 July, 9 days over 68 F
31 Aug-20 Sep, 21 days over 68 F

LGO, 1994


8 July-4 Aug, 28 days over 68 F
24 Aug-2 Oct, 40 days over 68 F


LWG, 1992
1 July, 1 day over 68 F
5 Aug-28 Aug, 25 days over 68 F
13 Aug-11 Sep, 29 days over 68 F

LWG, 1998


10 Jul-22 Jul, 5 days over 68 F
7 Aug-25 Aug, 7 days over 68 F
2 Sep-25 Sep, 24 days over 68 F

The general impression from these data is that maximum water temperatures have gone down since the reservoirs were created. The highest maximum temperatures (24.4°C or 76°F) generally occurred within the first year or two after the reservoir was created at all of the facilities except Lower Granite where in 1981 and 1990, 78°F and 77°F temperatures were recorded. Days when the maximum exceeded the standard started at Ice Harbor Dam as early as the 4th of July and as late as the middle of August. Temperatures exceeded the standard as late as the end of September. The duration of the exceedance appears to be more a function of the annual flow volume influenced by the duration of hot summer weather rather than related to water warming in the reservoirs.


Since 1991, cool water releases from Dworshak Reservoir have had a substantial impact on the maximum temperature and days of exceedance. From 1992 through 1998, the maximum temperature at Lower Granite Dam ranged from 68 to 72°F, whereas the average temperature since the dam began operating was 73.4°F, reaching a high of 77°F in 1990. The number of days of exceedance from 1992 to 1998 ranged from 0 to 36, whereas the average since the dam began operating was 44.3 days, ranging from 25 to 85 days.
Peak surface water temperatures at Lower Granite Dam during the sampled years were correlated to differences in average flow rates. For example, in 1977, average flow rates at Lower Granite Dam were below the historical average for most of the year. In contrast, the surface water temperatures observed were consistently higher than those observed in the high flow years of 1975 and 1976, and reached a peak of nearly 25°C. Conversely, the average monthly flow rate in August 1997 was approximately 150% greater than the average monthly flow rate in August 1994. The peak occurred between July and September, when observed temperatures were generally lower than those observed in 1994. Surface water temperature data collected in Ice Harbor Reservoir showed a similar relationship to flow, with higher temperatures being observed during the low-flow years of 1977 and 1994.
Peak water temperatures also appear to be influenced by ambient air temperatures, solar radiation, and percentage of total discharge contributed from Dworshak Reservoir. The influence of air temperature on peak water temperature is notable for 1995, when water temperatures reached a peak value that was lower than those observed in both 1994 (when relatively large releases from Dworshak Reservoir were initiated) and 1997 (a high flow rate year). This lower peak temperature observed in 1995 is likely attributable to the cooler-than-normal mean monthly air temperatures observed between June and late September.
Flow rates also seem to affect the duration of elevated water temperatures. The slower flow rates and increased surface area of water within the impoundments can cause surface waters to reach higher maximum temperatures and then cool down more slowly in the fall (BPA, 1995). In reviewing the data, 1977 and 1994 clearly stand out as having two to three months with surface temperatures above 20°C (68°F), as compared to less than two months observed in other years. In addition, it took at least a week longer in 1994 for the water temperatures at SNR-108 to drop back below 20°C (68°F). This longer period of elevated temperatures or the delay in cooling would be expected to adversely affect fish migration patterns.
At station CLR-326, upstream of the Snake River confluence, peak temperatures remained below 20°C (68°F) throughout the sampling period. Surface water temperature data collected from various stations along the Lower Snake River during the low-flow year of 1994 indicates that a higher maximum temperature was reached in Lower Granite reservoir (24.3 C) and upstream at SNR-140 (23.8 C) than during 1997. Previous research has indicated that thermal stratification in the Lower Snake River impoundments does not appear to occur to any significant extent (Funk et al. 1985). However, in 1997, a high flow year, the maximum temperature difference between surface and bottom waters was 4.7°C (8.2°F). This was observed on August 11 at Station SNR-108 in the Lower Granite Reservoir, the deepest reservoir station with the largest temperature gradient between zero and 10 m depth.
Temperature profile data from 1994, a low-flow year, depict a larger difference between surface temperatures and temperatures at depth following 28 days of large water releases from Dworshak Reservoir. The reservoir does not stratify, but may grade due to the sinking of more dense cooler water. This is most apparent in Lower Granite Reservoir (e.g., SNR-108 and SNR-129) where the difference between surface temperatures and temperatures at 35 meters depth approached 9° C.
Historically, the maximum temperature difference reported for surface and bottom waters was 4.7°C (8.2°F), measured in 1977 at SNR-108 (a high-flow year), and more typical differences are around 2.0°C (4°F) (Funk et al. 1985). It is noteworthy that both the 1997 and 1994 data were collected during a period of large water releases from Dworshak Reservoir (20.4 kcfs in 1997 and 25+ kcfs in 1994). Thus, the significant inputs of cooler water from Dworshak Reservoir, and the higher than normal river flows may have resulted in the larger than normal temperature gradients that were observed at depth. The remaining reservoir stations generally had a difference of less than a 2 to 3°C (3.6 to 5.4°F) throughout the water column (Normandeau 1999a). An increase in thermal gradation could lead to lower dissolved oxygen levels in the deeper waters and increased nutrient releases into the water column from bottom sediments if anoxic conditions were to occur with prolonged gradation or the formation of stratification.
In 1994, releases from Dworshak Dam began early in July, reaching a maximum of 25 kcfs by mid-month, and were completed by the end of the month. During this time period, the median flow contribution from the Clearwater River accounted for 54% of the total inflow to Lower Granite Reservoir and as much as 65% of the total flow on one occasion. Based on temperature data collected from D.H. Bennett from the UI, these cold water releases resulted in a 5°C (11°F) drop in water temperature in July in Lower Granite Reservoir at a depth of 6 meters. Differences in surface temperature data collected in Ice Harbor Reservoir (SNR-18) before and after 28 days of releases from Dworshak Dam in 1994 were less pronounced, with a maximum difference of 2.2°C. Similarly, Karr et al. (1997) noted a decrease in temperature at mid-depth from 5.3 C (9.5 F) at Lower Granite Dam to 2.4 C (4.4 F) at the Ice Harbor Dam. Temperature reductions were noted throughout much of the water column, although a steep gradient was present near the surface (Karr et al. 1997). In 1995, releases began in mid-July and continued to the end of August. The maximum release rate was 13.8 kcfs and accounted for about 45% of the downstream flows, and a temperature drop of 3° C (5.5° F) at the Lower Granite Dam (Karr et al. 1997). A similar release pattern was conducted in 1997 as well. Under these flow release conditions, downstream temperatures were apparently lowered by up to 10°C (50°F) in the Clearwater River and only by up to 1 to 2°C (2 to 4°F) in Lower Granite Reservoir. The impact on water temperature is delayed, and reduced with increasing distance downstream from Dworshak Dam (Karr et al. 1997; Normandeau 1999a).
In contrast to water temperatures, the highest dissolved oxygen (DO) concentrations are typically observed during spring runoff and tend to decline with increasing temperature. The USGS data going back to 1975 indicate that low minimum DO levels of 2.3mg/L have been recorded below Lower Granite Dam (RM 106.5).
Peak water temperatures were measured on July 28, August 23, and September 9, 1971, and these sampling occasions were also three of the four lowest DO readings obtained that year.
At the Lower Snake River stations, DO concentrations were for the most part above 8.0 mg/L during 1997 except during one late-summer event at each station. The timing of the seasonal low level seemed to occur first upstream (SNR-140) and then progressively moved downstream. The average low concentration for these three Snake River stations during this one sampling event was about 7.0 mg/L. A review of data collected in other years, particularly during the historically low flow conditions in 1994, reveal only minor differences. During an early September sampling event, at Station SNR-108, the average DO concentration dipped to near 6.8 mg/L but remained above 8.0 mg/L for all other sampling events.

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