Lakes

Approximately 85-90% of  lakes worldwide are supersaturated with respect to CO₂. This supersaturation results in CO₂ release to the atmosphere, and with a flux that makes lakes an important component of the global carbon budget. With a mean concentration of 1000-1200 ppmv, CO₂ concentrations in lake waters can reach 10,000-100,000 ppmv (1-10% pCO₂ by volume).

Anoxic lake sediments are active sources of dissolved methane to the overlying water column and to the atmosphere.  In stratified waters, elevated levels of methane can exist at depth. Surface concentrations of CH4 in lakes are highly variable and can reach  2 μmol/L or more under favourable conditions. As an example, during winter under ice cover in Lake Baikal, dissolved methane concentrations reach as high as ~4 μmol/L (~3000 ppmv CH₄).

Rivers

Typical levels of pCO₂ can range from below atmospheric, for example in algal blooms, to values of 2000 ppmv and more (sometimes, upwards of 10,000 ppmv) due to changing solubility, decay of organics, oxidation of methane, precipitation of carbonates, input from anthropogenic runoff sources, etc. Dissolved CO₂ levels in rivers are often calculated from pH and Alkalinity measurements, however, small errors in pH measurement can result in substantial error in pCO₂.

Dam spillways and waterfalls can cause bubble entrainment that leads to gas supersaturations that are detrimental to fish health. Monitoring of TDG downstream of dam spillways is required in many areas due to the problem that gas supersaturation poses to fish and other aquatic life, and levels must be maintained below regulated thresholds.