Great observation! I might of learned me something today!
I'm thinking there's several factors at play such as, elevation, slope, resistance of terrain, depth and width are a few that come to mind. When you look at those reference points on a map there's quite a bit of distance between them for those factors to have an effect. Another thought is where the rivers converge the rate of flow is probably slowed due to the waters merging with one another.
Link to map I observed:
https://maps.waterdata.usgs.gov/mapper/nwisquery.html?URL=https://waterdata.usgs.gov/mo/nwis/current?type=flow&group_key=county_cd&site_no_name_select=siteno&format=sitefile_output&sitefile_output_format=xml&column_name=agency_cd&column_name=site_no&column_name=station_nm&column_name=site_tp_cd&column_name=dec_lat_va&column_name=dec_long_va&column_name=agency_use_cd
While processing all this another question came to mind, how much water depth would there be if inflow rate was less than the outflow? Seems like the river would go dry if that was the case.