This river is deepest here.

[keannu]

When you compare within one same thing or person, do you always omit "the" for the superlative? Is it strict? If so, what do you think is the reason?

1. I'm most comfortable when I'm alone -Not "the most comfortable"
2. This river is deepest here.- Not "the deepest"

[keannu]

[Tdol]

The deepest part of the river is here.

[keannu]

The deepest part of the river is here.

You mean both forms are possible? But the first form is descriptive in the complement part. I mean if "This river is the deepest here" is impossible, making the strict rule. My grammar book defines like that, so I doubt if it's strict.

[JMurray]

If you are comparing rivers you might say: "Of the many that we have crossed today, this river is the deepest". "Of all those we have crossed, this is the deepest river".

If you are talking about a single river: "The boat sunk here where the river is deepest", "The river is deepest here, where the boat sunk".
Often: "The river is at its deepest here".

But, as Tdol says: "The deepest part of the river is here".
or...
"The river is (at its) widest where it flows around this bend".
"This bend is the widest part of the river".

not a teacher

[keannu]

If you are comparing rivers you might say: "Of the many that we have crossed today, this river is the deepest". "Of all those we have crossed, this is the deepest river".

If you are talking about a single river: "The boat sunk here where the river is deepest", "The river is deepest here, where the boat sunk".
Often: "The river is at its deepest here".

But, as Tdol says: "The deepest part of the river is here".
or...
"The river is (at its) widest where it flows around this bend".
"This bend is the widest part of the river".

not a teacher

Thanks a lot, it may be bugging, but why do you think you can omit "the" for a single object? Is it because there is no other comparing objects, so the meaning of specific or unique "the" disappears? It's my guess, sorry it's because I need to explain the reason to my students.

[Raymott]

If you are comparing rivers you might say: "Of the many that we have crossed today, this river is the deepest". "Of all those we have crossed, this is the deepest river".

If you are talking about a single river: "The boat sunk here where the river is deepest", "The river is deepest here, where the boat sunk".
Often: "The river is at its deepest here".

But, as Tdol says: "The deepest part of the river is here".
or...
"The river is (at its) widest where it flows around this bend".
"This bend is the widest part of the river".

not a teacher

Yes, but the reason for the sudden inclusion of "the" is that you've changed the comparison from the river to its parts. You are no longer - as keannu says -- "comparing within one same thing or person".
A river has many parts, so one part has to be the deepest.
There is only one river, so it has to be deepest somewhere.