Aerial photos track glacial changes

The Sauk-Suiattle Tribe is monitoring the amount of sediment moving through the watershed as glaciers recede.

“The tribe is concerned that global warming is exacerbating the amount and timing of this sediment by exposing steep, loose material in the late summer – prime salmon spawning season,” said Scott Morris, the tribe’s water quality coordinator.

A main source of suspended sediment is Glacier Peak. The tribe has access to high-resolution Long Distance and Ranging (LIDAR) images taken by the U.S. Geological Survey (USGS) in 2014 and 2015. But LIDAR, which surveys topography with an airborne laser, is expensive.

The Sauk-Suiattle Tribe is using a newer, less-expensive technique called photogrammetry. Last summer, Morris and three USGS hydrologists took GPS measurements in the floodplain and up to 7,000 feet on Glacier Peak, marking the spots with bright rocks and logs that could be spotted from the air.

Photographs were taken during a brief flight with a high resolution camera along the upper Suiattle River corridor, as well as the upper reaches of Chocolate Creek and Dusty Creek and the proglacial zone in between those two creeks on the flank of the volcano.

“The images can be superimposed onto the original LIDAR to highlight areas that either have accumulated bedload or lost bedload,” Morris said.

They plan to repeat the survey in 2019, pending funding. The data will complement an ongoing partnership with USGS to measure the timing and magnitude of suspended sediment in the Sauk, Suiattle and White Chuck rivers.

“The aerial photography helps us understand how bedload is moving in the uppermost reaches of the watershed, which would be difficult to measure by more conventional methods given the remote, rugged location,” Morris said.

Farther downstream, tribal and USGS staff are building on an earlier study by measuring the sediment contributions from smaller sub-basins in the Suiattle watershed to examine the influence of land use and other, natural factors.

The crew set up automated water samplers, flow instruments and continuous turbidity sensors on the mainstem Suiattle and Downey Creek, which features an important chinook spawning reach. Field crews also are collecting suspended sediment samples and flow during storms.