The characteristic cation in sea water is the sodium ion, Na+.
The characteristic cation in fresh (ground) water is the calcium ion, Ca2+.
The relative amounts of sodium and calcium ions in river water depend on the nature of the river.
Rivers that form from water that has run over well-weathered rocks (diagram A) tend to have low levels of dissolved solids and more sodium ions than calcium ions. Such rivers are said to be 'precipitation dominated'.
Rivers that form from water that has run over mainly unweathered rocks (diagram B) tend to have high levels of dissolved solids and more calcium ions than sodium ions. Such rivers are said to be 'rock dominated'.
Rivers which suffer a high degree of evaporation (usually because they are in hot places) do not conform to this general pattern (diagram C). Such rivers tend to have very high levels of dissolved solids but usually have more sodium ions than calcium ions because most of the calcium ions have been precipitated out. Such rivers are said to be 'evaporation dominated'.