STABILITY OF SPAWNING BEDS 



Dislodgment of salmonid eggs and larvae from 

 spawning beds during high water has been de- 

 scribed by a number of workers (Hobbs, 1937: 

 Hutchinson and Shuman, 1942; Davidson and 

 Hutchinson, 1943; Withler, 1952; Needham and 

 Jones, 1959; Wickett, 1959). Estimates of mor- 

 tality rates from gravel movement have not been 

 made in most instances. Furthermore, little is 

 known about the interactions of factors creating 

 unstable conditions in spawning beds and causing 

 mortality rates to increase. 



( Ihanges in surface profile occur where a stream- 

 bed degrades or aggrades. It is possible, however, 

 for bed movement to occur without an associated 

 change in surface profile or gradient (Mackin, 

 1948). One important unsolved problem is to 

 determine if bed movement can cause appreciable 

 mortality where there is no associated change in 

 streambed gradient. 



The effect of the pool-riffle complex on the 

 capacity of streams to produce pink and chum 

 salmon is not yet well understood. Factors im- 

 portant in generating the pool-riffle complex 

 include debris in the high-flow channel (Bishop 

 and Shapley, 1963) and bends in the channel. 

 Shifts in position of debris create unstable condi- 

 tions in the spawning bed which could lead to 

 dislodgment of salmon eggs and larvae. 



METHODS 

 STUDY STREAMS 



Field studies described in this report were con- 

 ducted mostly in three streams located in the 

 Kasaan Bay region of Prince of Wales Island, 

 Southeastern Alaska: Harris River, Indian ("reek, 

 and Twelvemile Creek (fig. 4). Watersheds of the 

 study streams are precipitous. Soils are shallow 

 and underlaid with impermeable materials. Ex- 

 cept for muskegs, which are poorly drained areas, 

 the watersheds have a very low capacity to retain 

 water. Runoff is rapid, and peak discharges 

 occur within a few hours after the beginning of 

 heavy rainfall. These high discharges occur 

 mostly in autumn. 



The study streams occasionally freeze over in 

 winter, when water temperatures near 0° C. have 

 been recorded for as long as 6 consecutive days; 

 summer water temperatures rarely exceed 13° C. 

 (James, 1956). 



Adult pink and chum salmon usually enter the 

 study streams to spawn between mid-August and 

 late September. Spawning occurs mostly in 

 September. A large percentage of pink salmon 

 spawn in intertidal areas, where the density of 

 spawners is highest in most years. 



Harris River is the largest of the three study 

 streams. Salmon have access to about an 8-mile 

 section of the main stream and its North Fork. 

 Chum salmon were observed to spawn mostly in 

 the North Fork, but pink salmon exhibited a 

 marked preference for a %-mile section of the upper 

 intertidal zone. Discharge during the spawning 

 period commonly fluctuates between 22 and 1,800 

 cubic feet per second (c.f.s.). During autumn 

 storms, average daily discharge may approach 

 5,000 c.f.s. Width of intertidal spawning riffles 

 during low flow averages about 60 feet. Spawning 

 beds consist of materials mostly less than 4 inches 

 in diameter. 



Indian Creek is the smallest of the study streams 

 but exhibits pronounced fluctuations in discharge. 

 During the spawning period, discharge commonly 

 varies between 4 and 300 c.f.s. Average daily 

 discharge during autumn storms approaches 900 

 c.f.s. some years, and a peak instantaneous 

 discharge of 6,400 c.f.s. was recorded on one 

 occasion (McNeil, Shapley, and Bevan, 1962). 

 Pink salmon spawn in Indian Creek, primarily 

 in an intertidal section one-fourth mile long be- 

 ginning at the confluence with Harris River. 

 The average width of Indian Creek spawning- 

 beds at low flow is about 25 feet. Spawning 

 beds consist of materials mostly less than 6 

 inches in diameter. 



Twelvemile Creek has a more stable discharge 

 than either Harris River or Indian Creek. During 

 spawning, average daily discharge usually varies 

 between 12 and 300 c.f.s. During autumn 

 storms, average daily discharge rarely exceeds 

 600 c.f.s. Intertidal spawning areas average 

 about 45 feet wide during low flow, and spawning 

 beds consist mostly of materials less than 4 inches 

 in diameter, and contain a high percentage of 

 sand and silt. About 5 miles are believed ac- 

 cessible to salmon, but the distance has not been 

 measured. Heaviest densities of spawning pink 

 salmon have been observed in the intertidal 

 zone. Chum salmon, less abundant here than 

 pink salmon, commonly spawn in the intertidal 

 zone, too. 



SPAWNING BED ENVIRONMENT OF PINK AND CHUM SALMON 



501 



