 object containing the point features and stored in a
new column named <code>upDist</code>.</p>
<p>The upstream distances stored in <code>upDist</code> are used to
calculate the pairwise hydrologic distances used to fit spatial
stream network models in the 'SSN2' package. Do not modify the name
of the column in any way or the values the <code>upDist</code> column
contains. If the <code>upDist</code> or <code>length_col</code> columns already exist in <code>sites</code> and <code>overwrite = TRUE</code>, the