The rules for using English articles are idiomatically complicated, but to a first approximation, use the definite article (the) for a specifically identified item and the indefinite articles (a,an) for generic items. Consider the following:
We paddled our canoes on the Hudson River until we were about ten miles north of Poughkeepsie, where we celebrated the New Year by a campfire on the riverbank.
The campfire is any old campfire, nothing to distinguish it from any other, so the indefinite article is appropriate. In contrast, the riverbank you're talking about isn't just any old riverbank, it's one that confines a particular river, so the definite article is appropriate. Note that this is different from your original example -- you could be referring to any riverbank.
The narrative continues:
We stayed at a campsite just up the road from the campfire place.
Campfire first. In this sentence it take the definite article because the previous mention has lent the campfire specificity. It's now not just any campfire. It's the one mentioned in the previous sentence.
Road next. Maddeningly enough, this doesn't seem to be a reference to a particular road, so saying "just up a road" would be fine. But up the road is an idiomatic phrase that can mean some distance. (Go here for a discussion of up the road a ways.) Just as maddeningly, up and down aren't necessarily opposites with roads accompanied by definite articles. Consider the Grateful Dead song "Goin' Down the Road, Feelin' Bad".