A hydrologic and morphologic analysis of the Black Lake, Alaska

Abstract

The Chignik Regional Aquaculture Association (CRAA) is concerned that Black Lake, located on the south side of the Alaska Peninsula, is naturally degrading as a juvenile sockeye salmon nursery habitat. CRAA considers the low-water storage capacity in the Black Lake system to be a major threat to Chignik sockeye salmon production. A unique methodological design was developed to address the water capacity reduction in the lake. This included the use of hydrodynamic/sediment transport numerical models in order to enhance our understanding of the physical/hydrological changes occurring in the lake. Finally, predictions of the changes that may occur in the lake’s water capacity within 22, 57 and 100 years are provided. There are three possible causes for the low-water storage capacity found in the Black Lake, namely, sedimentation, low water influx and changes at the outlet geometry of the Black lake. This study examined all three causes. With respect to the sedimentation issue, it was found that the deposition of the incoming sediment, originated from Alec (Sqaw) River, into the lake over the past 50 years has not substantially affected the lake storage capacity. It reduced the lake storage capacity by only about 1.0 % for the period of 1950-2005. With respect to the lake water influx, results suggest that the lake receives sufficient water influx to be able to fill to its 1950s storage capacity. It was concluded that the reduction in the lake water storage is mainly attributed to changes in the lake outlet geometry. The lake outlet base-level has been dropped by at least 1.0 m compared to the 1950s base-level. This drop can be attributed to the high differential found in sediment transport rates between the lake exit or the entrance of the Black River and the confluence of the West Fork with the Black River. Prior studies have recorded that the sediment influx from the West Fork has been significantly high and has affected the longitudinal gradient of the Black River downstream of the confluence point due to the occurrence of significant sediment deposition. Because the sediment rates entering the Black River are low comparatively to the rates from the West iv Fork, the river tends to degrade its bed upstream from the confluence in order to reach to the same level of longitudinal slope with the downstream section. In a nut-cell, the drop at the lake’s outlet describes the response of the Black River to significant sediment influx from the West Fork. To increase the water storage capacity in the Black Lake system, the researchers’ recommendation is the raising of the lake outlet base-level by 1.0 m and the placement of a temporal sill (e.g. inflatable sill, or a broad crested weir) at the outlet of the lake. The temporal sill would allow an increase in the storage of the lake. This increase is quantitatively described for the first time in this report. The sill should be installed during low flow conditions and removed during high flows. The placement of the sill will help the lake to temporarily regain its storage capacity. To get a complete answer to the Black Lake problem, the authors suggest carrying out an experimental study by building a physical model for the lake and its inlets and outlet to evaluate the performance of the outlet structure under various flow conditions. The physical model study also will explore ways to control excess sedimentation from the West Fork. Our long-term predictions pinpoint that a complete destruction to the ecosystem of the lake can happen within a century if the problem is not immediately addressed. The lake may lose 79 % of its 1950s storage capacity within a 100 year period.