The cinema’s leading purveyor of Southern Gothic, Jeff Nichols, hands Matthew McConaughey his latest tour de force turn in “Mud,” a down and dirty if entirely-too-long mythic melodrama in the “Tobacco Road” tradition.

Nichols has cooked up an exotic stew that includes obsessive love, a woman unworthy of it, a criminal on the run and a Huck Finn coming of age tale set against a dying way of life in backwaters Arkansas.

Ellis (Tye Sheridan, terrific) is a poor child who lives on one of the last houseboats allowed on that stretch of shoreline. He and his buddy Neckbone (Jacob Lofland) have the run of the river, know their way around skiffs and boat motors. But Ellis’ daddy (Ray McKinnon), who fishes for a living, knows they’re one misstep from being kicked off the houseboat his wife inherited. And when Ellis and Neckbone stumble across a man named Mud hiding out in an abandoned cabin cruiser jammed high in a tree on a mid-river island, that mistake seems already made.

Mud (McConaughey) is in “a tight spot,” he drawls, “and could use a little help.” He’d like to fix up this boat, which the boys covet themselves. Mud, it turns out, is waiting on a woman, “a dream you don’t want to wake up from.” And since Ellis is in the middle of his first crush, he agrees to help Mud.

Reese Witherspoon is Juniper, a Britney-trashy bombshell who has lived off how well she fills out a pair of Daisy Dukes her whole life. Ellis becomes go-between for Mud and Juniper, and being a growing boy, learns hard life lessons from them both.

Nichols revels in the milieu here, capturing authentic Southern voices and classic, overheated Southern melodrama — revenge, betrayal, strained family ties and attitudes toward women that date from the time of Jezebel.

McConaughey has truly found his place within the film firmament in Southern Gothic movies like this one and “Killer Joe.” Mud is full of folk wisdom and singular in purpose. Michael Shannon plays a river diver and inattentive uncle to Neckbone.

It doesn’t trivialize “Mud” to label it Tennessee Williams lite. Nichols gets good performances out of one and all, but lets himself get so caught up in his sense of place that this potboiler hangs around more than a few minutes after that pot has come to a boil.

But “Mud” is a vivid reminder that for all the changes cable TV and Interstate highways have wrought, there still corners of the country we hear very little about, places with a voice, vibe and vigor that are still distinctly emphatically Southern.