Assessment of ecosystem ‘health’ traditionally uses structural indicators (e.g., biotic community composition) but often neglects indicators of ecosystem processes (e.g., decomposition rates), providing an incomplete picture of overall ecosystem condition. Efforts to establish a framework identifying thresholds of organic matter decomposition expected in relatively intact, moderately degraded, and severely degraded ecosystems have been stymied by limited geographic scope and variation in organic matter composition that preclude general use of the framework. The goal of the CELLulose Decomposition ExXperiment (CELLDEX) project is to compare riparian and in-stream organic matter decomposition rates (i.e., loss of tensile strength per unit time) across all of the Earth's major biomes using a standardized cotton strip assay. Using a ‘crowdsourcing’ approach with researchers from over 40 countries, CELLDEX involves the deployment of a standardized substrate assay in approximately 400 streams and their riparian zones, representing each of Earth’s major biomes, and spanning 140 degrees of latitude. Each partner is responsible for incubating standardized cotton strips in four reference streams for ~ 30 days near their home institutions and provide ancillary data on water temperature and chemistry. This simple, inexpensive assay omits intrinsic differences in organic matter (i.e., variation in chemical composition and physical structure), thus allowing the influence of extrinsic environmental factors (e.g., temperature, water chemistry, flow or precipitation regimes) to be better assessed. This assay also is sensitive to differences in environmental conditions at regional and watershed scales, including those associated with environmental degradation. Cellulose strips were deployed for approximately 30 days from mid-October to mid-November in four streams and adjacent riparian zones in central Utah, USA to measure rates of decomposition. Data in this resources reflect only data from the state of Utah, USA.