Peering into murky waters
Updated: May 15
By Molly Stroud, PhD Student, Virginia Tech Geosciences
As rivers flow, they carry large amounts of solid material in their water. This material, often referred to as ‘total suspended solids’ (TSS) or ‘suspended particulate matter’ (SPM), amongst other names, plays a key role in the global sediment cycle as well as in inland water systems. Humans have been actively changing sediment transport patterns and volumes through modification of the landscape. Structures such as dams impound massive quantities of sediment, while activities such as agriculture and deforestation can greatly increase sediment loads. These changes affect ecosystems, impact water quality, and can worsen relative sea level rise. Tracking and understanding these changes is key to being able to take action to prevent future harm.
The above image, from Syvitski et al.’s 2005 paper ‘Impact of Humans on the Flux of Terrestrial Sediment to the Global Coastal Ocean’, shows how coastal sediment load is affected by human activities.
Satellite remote sensing offers a unique way to observe TSS in inland water bodies such as rivers and lakes. In situ TSS measurements, which consist of collecting a water sample and obtaining the mass of sediment in that volume of water, cannot be obtained on large spatial and temporal scales. Satellite remote sensing offers a solution to this problem. Using the visible and near infrared (VNIR) portion of the electromagnetic spectrum, we can create algorithms to estimate TSS in inland waters. Sensors onboard satellites such as Suomi NPP, the Landsat series, and Sentinel-2 provide data that can be used for this purpose. In addition to satellite data, in situ measurements are needed to calibrate an algorithm in the region of interest.
The image above shows the Glen Canyon Dam in Arizona. What do you notice about the color of the water north of the dam versus south of the dam? What might this imply about the sediment in the water?
One persistent challenge in this field is creating a spatially transferable TSS algorithm. This is due to variations in sediment type and color, which reflect differently in the VNIR and make algorithm transferability difficult. This is one challenge that the Global Rivers Group is trying to solve using a variety of innovative methods, including sensor optimization and lidar data.
Syvitski, J. P. M., Vörösmarty, C. J., Kettner, A. J., & Green, P. (2005). Impact of Humans on the Flux of Terrestrial Sediment to the Global Coastal Ocean. Science, 308(5720), 376–380. https://doi.org/10.1126/science.1109454
Molly Stroud is a PhD Student at Virginia Tech, Department of Geosciences, Global Rivers Group.