Arrow Lakes Tributary Fish Migration Access

Author: Hawes K., D. Drieschner, R. Wagner

The focus of CLBMON-32 was to determine the impact of reservoir operations on the upstream passage of key fish species, particularly Rainbow Trout, into Arrow Lakes Reservoir tributaries. Included in the geographic scope of this program are fish bearing tributaries that flow into Arrow Lake Reservoir. The scope of the program includes both a gray literature review of all studies relating to spawning assessments conducted on Arrow Lakes tributaries and a physical assessment of passage conditions into reservoir tributaries under varying reservoir and stream flow conditions. The key fish species for this program is Rainbow Trout (Oncorhynchus mykiss), and, to a lesser extent, Kokanee (O. nerka), and bull trout.

Q.1. Does the operation of the Arrow Lakes Reservoir block or reduce upstream migration of focal fish populations in tributary streams?

The operations of the Arrow Lakes Reservoir can block or reduce upstream migration of fish populations in tributary streams. This reduced access is a result of low to very low stream flows being conveyed through wider, aggraded, more poorly defined, and braided channels over the drawdown zone. The dynamic channel form exhibited over the drawdown zone of many tributaries is the result of the high vertical fluctuation of the reservoir. When these drawdown zones are exposed and stream flows are low to very low, upstream fish migration can be blocked.

In the spring, stream temperature monitoring suggests that Rainbow Trout migration may not occur until mid-April and later when passage conditions are good. Good passage conditions continue through June and July and decrease into September with the onset of Kokanee migration and spawning. Kokanee migration access has been observed to be reduced in some tributaries in spite of reservoir levels being at or above the 434m soft operating constraint. In these instances, access impairments are the result of very low stream discharges on individual watercourses. Bull Trout migrations to spawning grounds can occur as early as April and continue to September. This temporal range makes it difficult to focus on a specific period to monitor tributary drawdown zone conditions for Bull Trout passage. The combined higher stream flows and reservoir levels during earlier tributary access would provide good passage conditions for Bull Trout. There are 28 tributaries known to be frequented by Bull Trout. Twenty one of the streams are in the upper Arrow, 2 are situated in the Narrows, and 5 occur in the north end of Lower Arrow. With the exception of Eagle Creek that has late summer passage concerns and occasionally Drimmie Creek that has the potential to develop a braided channel upstream of the reservoir, the balance of tributaries supporting Bull Trout are not likely to have passage concerns. These tributaries are generally larger (higher order and magnitude streams) or occur in the upper Arrow basin, where higher elevation watersheds (i.e. more prolonged snow melt) and more moist to wet climate regime sustain higher flows throughout the year.

Q.2 Are there significant reservoir elevation thresholds below which spawner access is impacted?

The greatest potential for impediments to Rainbow Trout migration in the early spring (late March-early April) occur when reservoir levels are below 429 m and stream flows are low to very low.

The 2010-2013 stream temperature monitoring suggests that spring Rainbow Trout migrations may not occur until mid-April or later. Under current operations, tributaries are generally accessible by Rainbow Trout by the second to third week in April. At that time average reservoir elevations are around 427 m, based hydrometric data over the period of record (1984-2012), and relative discharges are rapidly increasing from low to moderate. As a result, passage conditions improve quickly through April with no concerns for Rainbow migration. However, the risk for migration barriers increases if Rainbow migrations were initiated in late March to early April when flows are still low to very low.

During Kokanee migration and spawning, the current soft operating constraint of 434 m represents a key elevation threshold. Reservoir elevation passage thresholds are dependent on stream flows. The observed very low stream flows that occur late August early September on some of the key tributaries sometimes result in passage assess impediments when Arrow Reservoir was at 434 m.

Q.3. Do high stream flows in tributaries in the spring mitigate impacts of low reservoir elevations?

Spring surveys in 2013 were completed in mid-May during the period when Rainbow Trout migration and spawning are expected to be in progress. Consistent with observations from previous years, high stream flows in tributaries in the spring mitigated potential impacts of low reservoir elevations.

Q.1. Does the operation of the Arrow Lakes Reservoir block or reduce upstream migration of focal fish populations in tributary streams?

The operations of the Arrow Lakes Reservoir can block or reduce upstream migration of fish populations in tributary streams. This reduced access is a result of low to very low stream flows being conveyed through wider, aggraded, more poorly defined, and braided channels over the drawdown zone. The dynamic channel form exhibited over the drawdown zone of many tributaries is the result of the high vertical fluctuation of the reservoir. When these drawdown zones are exposed and stream flows are low to very low, upstream fish migration can be blocked.

In the spring, stream temperature monitoring suggests that Rainbow Trout migration may not occur until mid-April and later when passage conditions are good. Good passage conditions continue through June and July and decrease into September with the onset of Kokanee migration and spawning. Kokanee migration access has been observed to be reduced in some tributaries in spite of reservoir levels being at or above the 434m soft operating constraint. In these instances, access impairments are the result of very low stream discharges on individual watercourses. Bull Trout migrations to spawning grounds can occur as early as April and continue to September. This temporal range makes it difficult to focus on a specific period to monitor tributary drawdown zone conditions for Bull Trout passage. The combined higher stream flows and reservoir levels during earlier tributary access would provide good passage conditions for Bull Trout. There are 28 tributaries known to be frequented by Bull Trout. Twenty one of the streams are in the upper Arrow, 2 are situated in the Narrows, and 5 occur in the north end of Lower Arrow. With the exception of Eagle Creek that has late summer passage concerns and occasionally Drimmie Creek that has the potential to develop a braided channel upstream of the reservoir, the balance of tributaries supporting Bull Trout are not likely to have passage concerns. These tributaries are generally larger (higher order and magnitude streams) or occur in the upper Arrow basin, where higher elevation watersheds (i.e. more prolonged snow melt) and more moist to wet climate regime sustain higher flows throughout the year.

Q.2 Are there significant reservoir elevation thresholds below which spawner access is impacted?

The greatest potential for impediments to Rainbow Trout migration in the early spring (late March-early April) occur when reservoir levels are below 429 m and stream flows are low to very low.

The 2010-2013 stream temperature monitoring suggests that spring Rainbow Trout migrations may not occur until mid-April or later. Under current operations, tributaries are generally accessible by Rainbow Trout by the second to third week in April. At that time average reservoir elevations are around 427 m, based hydrometric data over the period of record (1984-2012), and relative discharges are rapidly increasing from low to moderate. As a result, passage conditions improve quickly through April with no concerns for Rainbow migration. However, the risk for migration barriers increases if Rainbow migrations were initiated in late March to early April when flows are still low to very low.

During Kokanee migration and spawning, the current soft operating constraint of 434 m represents a key elevation threshold. Reservoir elevation passage thresholds are dependent on stream flows. The observed very low stream flows that occur late August early September on some of the key tributaries sometimes result in passage assess impediments when Arrow Reservoir was at 434 m.

Q.3. Do high stream flows in tributaries in the spring mitigate impacts of low reservoir elevations?

Spring surveys in 2013 were completed in mid-May during the period when Rainbow Trout migration and spawning are expected to be in progress. Consistent with observations from previous years, high stream flows in tributaries in the spring mitigated potential impacts of low reservoir elevations.