An Overview of Kelt Migration in Regulated Rivers: Status, Knowledge Gaps, and Future Directions Toward Safe Downstream Passage at Hydropower Facilities

Figures & data

Figure 1. A summary of the five main themes and main focuses within each theme from the literature review, illustrated for a HPP (top view) with: a forebay (A), a nature-like fishway (B), a guidance structure (C), a dam with three turbines (D), and a technical fishway (E).

Figure 2. Overview of future research priorities for studies of kelts in regulated rivers. New studies should aim to support one or two important goals: improving design for guidance, protection, and passage at HPP and/or improving river and operational management at HPP (orange). To do so, there are still substantial knowledge gaps within fundamental fields (green). These can begin to be addressed with various tools, including tools used in laboratory and field settings (blue).

Table A1. An overview of the key aspects of each kelt study included in the review.

Study	Species and sample size	HPP(s), river, and country	Passage route(s)	Turbine types and guidance structure(s)	Study objectives	Themes	Relevant results
Arnekleiv et al. (2007) Peer-reviewed article	Salmo trutta (41)	Hunderfossen Power Station River Gudbrandsdalslågen, Norway	Ice spillway, several other spillways (bottom spill), turbines, and fish ladder	Kaplan (2) turbines Screen at turbine inlet (no other description provided)	1. Passage selection. 2. Effect of surface water release on descent. 3. Threshold discharge values for kelt descent.	T1, T2, T3, and T5	1. Did not use submerged turbine shafts. Instead, passed via spillways. 2. Majority descended during surface water release. 3. Threshold discharge value for kelt descent was between 1 and 4 m^3s^−1,
Babin et al. (2021) Peer-reviewed article	Salmo salar (45)	Mactaquac generating station Saint John River, Canada	Spillway (bottom spill), diversion sluiceway, and via turbines.	Six Kaplan turbines Trash racks (16.5 cm spacing)	1. Over-winter survival. 2. Migration timing. 3. Migration rates through another river reaches. 4. Survival rates during reservoir and dam passage.	T1, T2, and T5	1. 56% and 75% Overwinter survival in years 1 and 2 of study period, respectively. 2. All tagged kelts experienced delay in forebay. 3. 44% unable to find open spill gates to pass HPP. 4. Migration rates were slower through reservoir than up or downstream. 5. Faster migration rates when water temperatures were higher. 6. Passage through reservoir caused 6-day delay in migration. 7. Swam near the surface 96% of the time; dives greater than 5 m only occurred near/in reservoir.
Baktoft et al. (2020) Peer-reviewed article	Salmo salar (48)	Svorkmo Power Station River Orkla, Norway	Fish ladder and spill gates	Two Francis turbines. Submerged wall with two openings covered by rack (8 cm spacing) in front of turbine intake.	1. Activity-based energy expenditure of kelts during migratory delay in front of dam.	T2	1. Estimated energy expenditure in study area varied between 0.8 and 10 KjH^−1 and depended on kelt length
Calles and Greenberg (2009) Peer-reviewed article	348 Salmo trutta upstream migrating adults were tagged; unclear how many survived spawning and became kelts	Finsjo hydropower station. Has two powerhouses: HEP2 and HEP3 River Emån, Sweden	HEP2: nature-like fishway and trash gates HEP3: nature-like fishway and trash gates	HEP2: one Kaplan turbine HEP3: two twin-Francis turbines HEP2: trash rack (30 mm spacing) and 1-m deep trash deflectors HEP3: trash rack (20 mm spacing) and 1-m deep trash deflectors	1. Effects of new nature-like fishways on migration.	T1 and T3	1. 28% migrated in the autumn and 78% migrated in the spring. 2. Migration occurred at roughly same temperatures, but discharge was higher in spring. 3. In the first year, all passed via nature-like fishway. In second year, migrated via trash gates and nature-like fishway.
Calles et al. (2012) Peer-reviewed article	Salmo salar (6) and S. trutta (14)	Herting hydropower dam. This HPP has two powerhouses River Ätran, Sweden	Denil-fishway, a juvenile eel pass, spill gates, and a bottom siphon	Two Kaplan turbines at first powerhouse; one Kaplan turbine at second powerhouse There is a rack in front of the intake for the first powerhouse (40 mm spacing). There is a rack at a turbine intake further downstream (90 mm spacing angled at 60 degrees) At the other powerhouse, there is one rack (40 mm spacing angled at 77 degrees). A second rack (22 mm spacing) is placed in front of the first rack in spring	1. Efficiency of racks for guidance. 2. Route selection. 3. Residence time. 4. Upstream of HPP. 5. Overall passage success.	T2 and T3	1. Passage success inversely related to body length. 2. Trout had highest guidance efficiency. 3. Trout had a longer residence time before passage than salmon. 4. Most kelts “searched” between the two powerhouses at the HPP. 5. Five salmon kelts passed the HP - one went via surface spill, one via bottom spill, and three via turbines. 6. Nine trout passed HPP - seven went via surface spill, one via bottom spill, and one via turbines. 7. Three salmon and one trout died on racks.
Colotelo et al. (2013) Technical report	Oncorhynchus mykiss (324)	Lower Granite, Little Goose, Lower Monumental, Ice Harbor, McNary, John Day, The Dalles, and Bonneville Dams Columbia and Snake rivers, USA	Spillway weir, traditional spill, turbine, and juvenile bypass system	Four dams have 6 turbines, 2 have 16 turbines, 1 has 18 turbines, and 1 has 22 turbines. Information on the types of turbines is not specified No information about guidance structures specified	1. Annual kelt population abundance at Lower Granite Dam. 2. Route survival and passage probabilities at each dam. 3. Forebay residence time, tailrace egress time, and time to pass from forebay to tailrace exit.	T2, T3, and T5	1. 40.7% survival rate. 2. Most frequently passed via spillway routes. 3. Survival estimates varied by dam. 4. Low passage via turbines (1.5 − 6.5%). 5. Residence, tailrace egress, and forebay to tailrace times decreased as the kelts moved downstream past each dam. 6. Median forebay residence time were less than 1.5 h at all dams. 7. Median tailrace egress time was less than 0.5 h at all dams. 8. Median time to pass from forebay to tailrace exit was 2.5 h at all dams.
Colotelo et al. (2014) Technical report	Oncorhynchus mykiss (487)	Lower Granite, Little Goose, Lower Monumental, Ice Harbor, McNary, John Day, The Dalles, and Bonneville dams. Columbia and Snake rivers, USA	Spillway weir, traditional spill, turbine, and juvenile bypass system	Four dams have six turbines, two have sixteen turbines, one has eighteen turbines, and one has twenty-two turbines. Information on the types of turbines is not specified No information about guidance structures specified	1. Route-specific selection and survival probabilities. 2. Forebay residence time, tailrace egress time, and time to pass from forebay to tailrace exit. 3. Annual population abundance at each dam.	T2, T3, and T5	1. 27.3% survived passing all dams. 2. Largest proportion passed via spillways instead of juvenile bypass systems or turbines. 3. Median forebay residence time less than 2 h at all dams. 4. Median tail egress time was approximately 0.5 h at all dams. 5. Median time to pass from forebay to tailrace was less than 3.1 h at all dams. 6. Approximately 11 750 kelts arrived at the dam the furthest upstream.
Evans et al. (2008) Peer-reviewed article	Oncorhynchus mykiss (5878)	Lower Granite, Little Goose, Lower Monumental, Ice Harbor, McNary, John Day, The Dalles, and Bonneville dams Columbia and Snake rivers, USA	Transportation via barge or truck or passage via dams (dam routes not specified)	Turbine type not specified Fish screens at Lower Granite and John Day Dam	1. Evaluate use of transportation to increase repeat spawner return rate	T1 and T4	1. 1.9% of kelts were detected as repeat spawners. 2. Kelt condition was best predictor of repeat spawner return rate. 3. Female and wild kelts were more likely to return as repeat spawners than males or hatchery fish. 4. Early migrating kelts were more likely to return as repeat spawners than late migrants. 5. Smaller kelts were more likely to return as repeat spawners than large kelts. 6. Kelts transported via barge/truck past HPP dams were more likely to return as repeat spawners than kelts that migrated in the river.
Ferguson et al. (2005) Technical report	Oncorhynchus mykiss; sample size not specified.	Lower Granite, Little Goose, Lower Monumental, McNary, Ice Harbor, John Day, The Dalles, and Bonneville Dams Snake and Columbia rivers, USA	Spillway weir, traditional spill, turbine, and juvenile bypass system	Not specified	1. Synthesis of several technical reports about kelt passage through various dams.	T3	1. Passage selection and FGE varies based on the HPP and the passage routes available.
Ferguson et al. (2008) Peer-reviewed article	Salmo salar and S. trutta Sample size not applicable. Model is parameterized with kelt length data from several other studies.	Sikfors Power Station and Stornorrfors Power Station Rivers Piteälven and Umeälven Sweden	Sikfors Power Station: fish ladder Stornorrfors: no fish passage solution	Sikfors: two Kaplan turbines No information about guidance structures provided Stornorrfors: four Francis turbines Trash racks (10 cm spacing) present upstream of turbines	1. Combine blade-strike models with lifecycle models to analyze effect of HPP on fish populations.	T4	1. Estimated mortality between 25.2% and 45.3%. 2. No difference in estimates of mortality from Kaplan or Francis turbines. 3. Highest population growth rates occur when both smolts and kelts are protected from blade strikes.
Fuchs et al. (2021) Peer-reviewed article	Oncorhynchus mykiss (174)	The number of HPP a kelt passed varied based on the tributary in which it overwintered, but they could have passed up to 9 dams before the Bonneville Dam. Columbia River, USA	At Bonneville Dam, juvenile bypass system and spillways	Not specified	1. Estimate survival during spawning period 2. Monitor kelt movements from tributaries and estimate migration survival.	T1 and T4	1. 56% of steelhead survived to spawn. 2. Downstream survival of wild kelts was estimated at 65.1% during first year of study and 22.9% during second year. 3. Downstream survival of hatchery-reared kelts was 22.1% in first year (no hatchery kelts tagged in second year).
Hatch et al. (2003) Technical report	Oncorhynchus mykiss (210)	Lower Granite, Little Goose, Lower Monumental, Ice Harbor, McNary, John Day, The Dalles, and Bonneville dams. Columbia and Snake rivers, USA	Spillway weir, traditional spill, turbine, and juvenile bypass system	Number and type of turbines not specified Diversion screens in turbine intakes guide fish to bypasses present at all dams except the Dalles Dam	1. Migration rates, passage routes, and survival	T3	1. 13% of tagged kelts passed all dams. 2. Fish condition positively correlated to passage success. 3. 73% passed via spillways. 4. Suggest turbine passage is reduced when spill is provided.
Keefer et al. (2018) Peer-reviewed article	Oncorhynchus mykiss (107)	Lower Granite, Little Goose, Lower Monumental, Ice Harbor, McNary, John Day, The Dalles, and Bonneville dams Columbia and Snake rivers, USA	Not specified	Not specified	1. Assess cumulative survival after spawning. 2. Assess cumulative survival during kelt migration past dams.	T1 and T4	1. Minimum kelt survival from Lower Granite reservoir to Ice Harbor Dam was between 0.31 and 0.39. 2. Minimum kelt survival from Ice Harbor Dam to Bonneville Dam was 0.125–0.203 3. Higher survival rates for kelts that passed fewer dams. 3. Survival between Lower Granite reservoir and Ice Harbor Dam was lower for 2-SW kelts than other kelts. 4. Survival between Ice Harbor Dam and Bonneville Dam decreased as kelt length increased.
Kraabøl et al. (2008) Peer-reviewed article	Salmo trutta (41)	Hunderfossen Power Station River Gudbrandsdalslågen, Norway	Turbines, a fishway, and spillways	Two Kaplan turbines Screen at turbine inlet (no other description provided)	1. Influence of sex and body length on emigration.	T1	1. 49% migrated in autumn; 51% overwintered in the river. 2. Females five times more likely to migrate in the autumn, and eight times more likely to descend the spillways during the first release of surface water. 3. Bigger kelts migrated earlier than smaller kelts.
Leander et al. (2024) Peer-reviewed article	Salmo salar (31) and S. trutta (9)	Stornorrfors Hydropower Station River Umeälven, Sweden	A side channel in the river, allowing the kelts to bypass the HPP,	Turbine types not specified Bubble barrier	1. Quantify guidance efficiency of bubble barrier.	T3	1. Sonar and telemetry data showed kelts were significantly more likely to use side channel when the bubble barrier was in use.
Nyqvist et al. (2016) Peer-reviewed article	Salmo salar (97)	Edsforsen, Skoga, Krakerud, Forshult, Skymnäs, Munkfors, Deje, and Forshaga power plants River Klarälven, Sweden	Spill ways and turbines	Five HPP have two Kaplan turbines each, one has three Kaplan turbines, one has one Kaplan and two propeller turbines, and one has a H-Francis turbine, a Kaplan turbine, and two propeller turbines Edforsen: intake rack (70 mm spacing). No information about guidance structures specified for other HPP.	1. Migration timing and speed. 2. Behavior at the Edforsen HPP. 3. Postspawning survival. 4. Accumulated survival during consecutive dam passage.	T1, T2, T3, T4, and T5	1. 49% survived spawning and initiated migration. 2. Females and small fish had higher post-spawning survival. 3. Migration rates were variable in non-impounded river reaches. 4. Most kelts passed Edforsen via upward opening spill gates. 5. Median residence time in Edforsen was 25 min. 6. None of the tagged kelts survived passage of all eight HPP.
Nyqvist, Bergman, et al. (2017) Peer-reviewed article	Salmo salar (40)	Edsforsen, Skoga, Krakerud, Forshult, Skymnäs, Munkfors, Deje, and Forshaga power plants River Klarälven, Sweden	Spill ways and turbines	Five HPP have two Kaplan turbines each, one has three Kaplan turbines, one has one Kaplan and two propeller turbines, and one has a H-Francis turbine, a Kaplan turbine, and two propeller turbines Edforsen: intake rack (70 mm spacing) No information about guidance structures specified for other HPP	1. Movements in forebay and intake area of Edforsen HPP. 2. Passage route selection and route-specific survival.	T3, T4, and T5	1. 41% passed Edforsen dam. 2. 0% mortality rate of kelts that passed via spillways; 62% mortality of kelts that passed via turbines. 3. Primarily surface oriented in forebay. 4. More likely to pass via turbines at night. 5. Only one fish survived migrating past all eight HPP.
Nyqvist, Nilsson, et al. (2017) Peer-reviewed article	Salmo salar (115)	Herting hydropower station. This HPP has two powerhouses. River Ätran, Sweden	Turbines, spill gates, Denil fishway, surface spill bypass, and a nature-like fishway	First powerhouse: two Kaplan turbines. Upstream rack (40 mm spacing) and turbine intake rack (90 mm spacing and 60-degree angle) Second powerhouse: one Kaplan turbine Turbine intake rack (40 mm spacing) overlayed by second rack (22 mm and 77-degree angle)	1. Passage rate, passage efficiencies, and delay before and after construction of nature-like fishway and new low sloping racks	T2, T3, and T5	1. Passage rate increased, and delays decreased, after construction of new bypass. 2. Before modifications, discharge affected passage performance. Effect of discharge no longer significant after remedial measures.
Scruton et al. (2007) Peer-reviewed article	Salmo salar (103)	Bishops Falls hydroelectric generating facility Exploits River, Canada	Spillways, via a smolt opening, over the dam, via reverse fishway, downstream fish bypass, and via turbines.	Number and type of turbines not provided No guidance system	1. FGE. 2. Residency time in forebay. 3. Passage survival rates.	T2, T3, and T5	1. Mean FGE was 92%. 2. 92% Went via bypass, 7% went via turbines. 3. 0% Survived turbine passage. 4. Selected route within one hour of being released into forebay.
Scruton et al. (2008) Peer-reviewed article	Salmo salar (107)	Grand Falls-Windsor and Bishops Falls hydroelectric facilities Exploits River, Canada	Grand Falls: bypass or via turbines. Bishops Falls: spillways, via a “smolt” opening, over the dam, via reverse fishway, downstream fish bypass, and via turbines	Grand Falls: five Francis turbines Floating louver Bishops Falls: number and types of turbines not specified No guidance system	1. Review the results of various studies at the two HPP. 2. How improvement for downstream passage at these facilities have improved overall passage success.	T3	1. FGE varied interannually but improved with improvements to guidance structures. 2. Poor guidance from floating louver. 3. Moved through forebay quickly, suggesting they quickly found bypass entrance.
Trammell et al. (2016) Peer-reviewed article	Oncorhynchus mykiss (7410)	McNary, John Day, and Bonneville dams Yakima River, Columbia River, USA	Not specified	Not specified	1. Evaluate the effects of transport past HPP (truck) and reconditioning (fed and treated for infections at a hatchery) on steelhead survival.		1. 0.9% of kelts that were transported, but not reconditioned, returned as repeat spawners. 2. 3.2% of kelts that were reconditioned and then transported returned as repeat spawners. 3. 11.5–17.6% of kelts that were reconditioned long-term became repeat spawners. These fish did not have to pass HPP.
Vikström et al. (2020) Peer-reviewed article	Salmo salar (97) and S. trutta (10)	Stornorrfors Power Station River Umeälven, Sweden	Only studied kelts that passed via turbines	Four Francis turbines No information about guidance structures specified	1. Validate accuracy of mortality estimates from blade strike models.	T4	1. Model underestimated mortality of passing Francis turbines. 2. Model did not reflect negative correlation between mortality and discharge observed in field data. 3. Mortality rate of 56-81% for kelts passing Francis turbines.
Weiland et al.(2009) Technical report	Approximately 395 Oncorhynchus mykiss kelts, based on body length data for fish passing dam	Bonneville Dam Columbia River, USA	Only studied kelts that passed via the surface flow outlet and juvenile bypass system	Eighteen turbines. Type not specified Submersible traveling guidance screens	1. Hourly downstream passage rate of kelts migrating via surface flow outlet. 2. Behavior upstream of surface flow collector and during passage.	T5	1. No distinct vertical distribution was observed, though kelts seemed skewed toward the middle and surface of the water column. 2. Either passed quickly in surface flow outlet or moved back upstream. 3. Oriented tail-first was passing HPP, which was suggested to be a response to the higher flows in front of surface flow outlet and intake.
Wertheimer and Evans (2005) Peer-reviewed article	Oncorhynchus mykiss (853)	Lower Granite, Little Goose, Lower Monumental, McNary, John Day, The Dalles, and Bonneville dams Columbia and Snake rivers, USA	Spillways, via turbines, and bypass channels	Number and type of turbines not specified Diversion screens in turbine intakes are present at all dams except the Dalles Dam	1. Migration rates, passage routes, and passage success. 2. Compare results from a low-flow year and a normal-flow year.	T2, T3, T4, and T5	1. Migration success past all dams was poorer during low-flow year (4.1%) than regular-flow year (15.6%). 2. Primarily passed via spillways or surface flow routes. 3. During spill, 90% passed via non-turbine routes. 4. Turbine passage might be substantial source of mortality in non-spill periods.
Wertheimer (2007) Peer-reviewed article	Oncorhynchus mykiss (842)	Bonneville Dam. This HPP has two powerhouses Columbia River, USA	via turbines, sluiceway, spillways, the juvenile bypass system, and the surface flow outlet	Eighteen turbines. Type not specified Screens in front of turbines (spacing not specified)	1. Forebay residence of kelts passing surface flow outlet. 2. Passage efficiency of surface flow bypass.	T2, T3, and T5	1. Passage time and migration delays reduced with improved surface flow outlet. 2. Passage efficiency improved with new surface flow outlet. 3. Approximately 80% were successfully diverted from turbines.
Östergren and Rivinoja (2008) Peer-reviewed article	Salmo trutta (58)	Sikfors and Stornorrfors power stations Rivers Vindelälven and Umeälven, Sweden	Spillways (bottom spill), fish ladders, and turbines	Stornorrfors: four Francis turbines Sikfors: two Kaplan turbines No guidance devices at either station	1. Migration and overwintering strategies. 2. Migration timing of kelts. 3. Passage success at HPP.	T1, T2, and T4	1. 91% survived spawning. 2. 8% overwinter mortality rate. 3. Main springtime migration occurred in May and June. 4. Passage mortality was 69% at Sikfors HPP and 25% at Stornorrfors HPP. 5. One kelt died while passing via a spillway.

The study objectives listed in the table pertain to objectives specific to kelts. The main themes identified in the studies are: postspawning survival and migration initiation (T1), migration delays before dam passage (T2), passage selection and fish guidance efficiency (T3), passage mortality (T4), and swimming behaviors (T5).

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