Geographic and ecological nexus

Dead center of Greater Chilkat Watershed, Little Salmon [noTN?] is a spatial and ecological nexus, where energy, structural diversity and hydrocomplexity crests.

WSW upvalley from July drone flight, when haze from Canadian fires obscured the horizon. On left is Gathéeni (Tsirku) and on right is a prominent landslide. Our bedrock map indicates this occurred on Mississippian carbonaceous phyllite. Already present on 1929 air photos, it’s so active that scree can’t revegetate.

I first heard about the Little Salmon in the late 1980s, working with consulting hydrologist Dan Bishop on a study of Chilkat Lake outlet for Northern Southeast Regional Aquaculture Association. Part of the puzzle of the lake’s reversing outlet/inlet is tied to an upriver bedrock constriction of Gathéeni, sockeye stream (Tsirku) at Eey X’é, mouth of rapids (Devil’s Elbow).

Navy stereopair for the ‘unhealable’ slope failure on karst ‘nose’ between L’ehéeni and Gathéeni. In upper right corner is a glimpse of Klehini river and the miner’s road to Porcupine.

Dan explained the Elbow’s bottleneck effect was so strong that glacial Gathéeni ‘leaked’ northward, mostly as groundwater, through a 3-mile-long curving bottomland ending in Little Salmon marshes—no doubt an ancient pathway of the river’s main channel. At the time I couldn’t map contours in this valley, and had to take Dan’s word for it.

Thirty years later, I began to hear from Jessica Plachta and Nicholas Szatkowski about Little Salmon marshes’ unique hydrology, where open water is found even in coldest weather. Was there some connection to Dan’s proposed Tsirku River ‘detour?’

Generating 5-meter contours from ArcticDEM (no LiDAR yet for this area) strongly supports this idea. The Tsirku ‘detour-leakage’ drops about 30 meters in 4 kilometers from Eey X’é through ancient alluvium, likely threaded by braiding paleochannels of the historic Gathéeni. That’s a gradient of 41 feet per mile.

In comparison, downriver from Eey X’é on Gathéeni’s contemporary channel, the river drops those same 30 meters in only 3.6 km, for a translated gradient of 46 feet per mile. Clearly, flow will ‘prefer’ this more efficient route until gradients change through incision or deposition.

Download 9MB geopdf here): †PDF_Download

Hillshade based on 2m-pixel ArcticDEM (actually a veg-surface model, or DSM). Dated cutting units in red. Arrows show view of drone oblique. This can be downloaded as a geopdf for field navigation.