This does look like a Ruellia (a Petunia cousin), yes, though we cannot see enough detail to identify it to species. Four species are documented in MD, though the most common of the group is Carolina Wild Petunia (Ruellia caroliniensis).
http://www.marylandbiodiversity.com/viewChecklist.php?genus=Ruellia

Carolina: http://www.marylandbiodiversity.com/view/2181

Miri

A combination of traits can help narrow-down the possibilities of which species this, including:

• plant height (which is of course influenced by plant age and time of year, so not a concrete trait)
• growth habit
• relative leaf and stem hairiness
• calyx characteristics (the leaf-like lobes holding the base of the flower), though the relative length of such, despite being a defining character for caroliniensis, is difficult to assess as others look quite similar
• habitat/location found (we see you tagged Montgomery County, but was this seen there or elsewhere?)

This North Carolina page discusses some comparative characteristics of this species with others that are also found in MD: https://auth1.dpr.ncparks.gov/flora/species_account.php?id=4

In the National Wildlife Federation book Field Guide to Wildflowers of North America, Ruellia humilis is noted as having stalkless leaves and Ruellia strepens as having nearly hairless stems when compared to Ruellia caroliniana.

Here are other entries for the MD species, though identifying details vary in availability from source to source:

• https://www.wildflower.org/plants/result.php?id_plant=RUCA4
• https://www.wildflower.org/plants/result.php?id_plant=RUHU
• https://www.wildflower.org/plants/result.php?id_plant=RUST2
• https://www.illinoiswildflowers.info/woodland/plants/sm_petunia.htm

Minor correction of prior answer: Ruellia is not actually a Petunia cousin - they are in different plant families - but share the name based on their general appearance.

Miri

You're welcome. Identifying closely-related plants to species level can sometimes be challenging, given that some details are not only variable from individual to individual, but also may overlap slightly (or be difficult to discern) between species. It's likely the plant you encountered is Ruellia caroliniana given its relative abundance in the area in general, plus along the canal and river in the Great Falls area.

Miri