When you look at a river from above—what we call its "planview geometry"—you're essentially peering into a wealth of information about that river system. It's like a fingerprint, unique to each river, revealing intricate details about its life story. This overhead view can tell us about the river's morphological evolution, how water and sediment move through its channel, and much more.
Hydraulic Geometry and Rating Curves
One well-established theory in fluvial geomorphology is that of hydraulic geometry. It suggests a power-law relation, known as a "rating curve," between a river's width and discharge. So, if you know the width of a river at a certain point, you can make educated guesses about its discharge level. This is a powerful insight that has modern applications. For instance, remote sensing images can measure a river's width and, consequently, monitor its discharge. This is particularly useful in regions where in-situ measurements are difficult to obtain.
Meandering and Morphological Changes
Rivers are not static; they are more like living organisms that evolve. As water moves from higher elevations to lower ones, rivers adjust their courses through a process known as meandering. This leads to lateral shifts in the river's course, driven by the erosion and accretion of sediment along the banks. Understanding these changes is essential for various applications, from flood management to environmental conservation.
The Technological Evolution: From Aerial Imagery to Earth Engine
Early Days: Manual Interpretation
Scientists relied heavily on aerial imagery in the early stages of studying rivers. These images were manually interpreted to deduce the characteristics of river systems, a labor-intensive and time-consuming process. But it was a start, giving us invaluable insights into river morphology.
The Digital Revolution: Automated Processing
Fast forward to the 21st century, and we're growing exponentially in computing power and data availability. The RivWidth software (Pavelsky and Smith 2008) first presented an automated method for river width extraction, and RivWidthCloud (RWC) (Yang et al. 2019) later applied and expanded these methods to Earth Engine. Similarly, methods for detecting changes in river form have evolved from simple tools that track hand-drawn river centerlines (Shields et al. 2000) to automated methods that can process entire basins (Constantine et al. 2014, Rowland et al. 2016).
Modern Capabilities: Large-Scale, Cloud-Based Analyses
As technology advanced, methods for detecting changes in river form have also evolved. We've gone from simple tools that relied on hand-drawn river centerlines to sophisticated automated methods capable of processing entire river basins. The marriage between advanced computing and fluvial geomorphology has been happy, leading to the development of many Earth Engine-based tools for large-scale, cloud-based analyses.
The Present and Beyond: Unleashing the Power of Earth Engine
The capabilities of Earth Engine have dovetailed nicely with the needs of modern fluvial geomorphology. This platform allows for large-scale, high-speed data analysis that would have been unthinkable just a few years ago. With Earth Engine, researchers and policymakers now have a powerful tool for understanding and managing one of the planet's most vital natural resources: its rivers.