what is the meaning of River Mouths in the LOSC and law of the sea

Concerning river mouths, Article 9 of the LOSC stipulates:
If a river flows directly into the sea, the baseline shall be a straight line across the mouth of the river between points on the low-water line of its banks. The language is almost identical to Article 13 of the TSC. This provision calls for four brief
comments.
First, concerning the interpretation of the phrase ‘directly’, the authentic French text of Article 9 reads: ‘si un fleuve se jette dans la mer sans former d’estuaire’. The French text clearly suggests that the phrase ‘directly’ means ‘without forming an estuary’. It follows that a river under Article 9 is a river without an estuary. In reality, it may be difficult to distinguish between a river that flows directly into the sea and one entering the sea via an estuary.

A second issue pertains to the selection of the base points of a straight line across the mouth of the river. Apart from the general requirement that the base points must be on the low-water line of the river bank, there is no further specification in Article 9. However, the mouth of the river can be difficult to locate, particularly on a coast with an extensive
tidal range.

Third, Article 9 specifies no limitation on the length of the line across the mouth of the river. It may also be noted that the straight line across the mouth of the river shall either be shown on charts or the coordinates of the ends of the lines must be listed pursuant to Article 16 of the LOSC.

Finally, according to one view, Article 9 would appear to apply both to rivers with a single riparian State and to rivers with two riparian States. However, the act of drawing baselines is necessarily a unilateral act. It is debatable whether a coastal State can unilaterally draw a straight line across the mouth of the river from or to a base point located in another coastal State, without the agreement of that State. In practice, the United States protested the closing line of the Rio de la Plata drawn by Argentina and Uruguay on the ground that Article 13 of the TSC does not apply to rivers whose coasts belong to two or more States.