The surfaces of rivers are hotspots for biogeochemical exchange and emit significant amounts of greenhouse gases globally. Estimates of river surface area are critical to determining fluvial greenhouse gas evasion yet are currently poorly constrained. The relative abundance of narrow rivers to wide rivers is commonly assumed to be fractal, or scale invariant. This assumption aids in statistical estimates of river surface area but has not been tested across spatial scales. We measured river size in four nested basins within the Mississippi River Basin using a combination of remote sensing and field surveying to determine the statistical size distribution of rivers from continental to headwater scales. We found that the relative abundance of narrow rivers to wide rivers consistently fits a log-normal probability density function, supporting the assumption of fractal river size. Using the fractal size distribution of rivers, we estimated a total river surface area of 17,828 ± 5,126 km2 (0.54 ± 0.16 % of land surface area) in the Mississippi River Basin which is comparable to previous studies. Our multi-scale approach revealed the fractal nature of river size and allowed for a more accurate accounting of river surface area with implications for the role of rivers in biogeochemical cycling. / Master of Science / Greenhouse gases and other gaseous chemicals enter the atmosphere at the surfaces of rivers. Knowing the total surface area of rivers can help us figure out how much greenhouse gas comes from rivers, but there is a wide range of estimates for the total river surface area. Some estimates of total river surface area rely on the assumption that river networks look similar when zoomed in or zoomed out, like the leaves of a fern or the crystals in a snowflake. To test this assumption, we measured the size of rivers in the Mississippi River Basin at large and small scales using satellite and aerial imagery as well as by hand in the field. We found evidence that supports the assumption that river networks look similar at all scales. We also used our measurements to make a statistical estimate of total river surface area in the Mississippi River Basin: 17,828 ± 5,126 km2, or 0.54 ± 0.16 % of the total land surface area, which is a similar percentage to what other researchers have found. This approach gave us a clearer picture of the surface area of rivers of all sizes which is important because small rivers emit greenhouse gases at a faster rate than large rivers.