Learn German: Hier ist die Strömung am schnellsten.

Yes, you can! die Schnelle is a valid noun meaning "rapidity" or "quickness." However, to express the general concept of "speed," German speakers usually use die Schnelligkeit (or die Geschwindigkeit). You will most often hear die Schnelle in the very common phrase auf die Schnelle, which means "in a hurry" or "on the fly."

Why is it am schnellsten and not just schnellst or das schnellste?

When a superlative adjective stands alone after the verb sein (to be) rather than right in front of a noun, you must use the am ...-sten construction. If you wanted to put the adjective directly in front of the noun, you would use an article and an adjective ending: die schnellste Strömung (the fastest current).

Strömung is feminine. Does it follow the same noun-creation rule as the words in this lesson?

It shares the feminine gender, but uses a different building block. The nouns focused on in this lesson are derived from adjectives (like groß -> die Größe). Strömung, on the other hand, is made by taking the stem of the verb strömen (to flow/stream) and adding the suffix -ung. Remember that all German nouns ending in -ung are strictly feminine.

Why is Hier at the start of the sentence instead of Die Strömung?