The dirty snow
As it retreats
Leaves small moraines
Upon the streets;

But melt that flows
Into the drains
Deposits eskers
Not moraines.

A moraine (as the Encyclopædia Britannica reliably informs) is an accumulation of rock debris that has been carried or shoved, then dropped or abandoned, by a glacier. A moraine is a jumble, for all it may deposited more or less neatly:

An esker (says, again, Encyclopædia Britannica) is a ridge deposited by a subglacial or englacial meltwater stream, with the deposited material generally sorted by grain size–the sort of attention to detail one would expect from flowing water. “Eskers may range from 16 to 160 feet (5 to 50 m) in height, from 160 to 1,600 feet (500 m) in width, and [from] a few hundred feet to tens of miles in length.” So:

A tip of the hat to James Harbeck at Sesquiotica, for his learned discourse upon the history and flavor of the word esker.

Images: Detail of Dirty Snow by Mark Turnauckas, published under a Creative Commons Attribution 2.0 Generic (CC BY 2.0); Glacier National Park, Montana. Terminal moraine at the foot of a small glacier on the north slope of Jackson Mountain. July 1914. – ID. Stebinger, E.C. 406 – sec00406 – U.S. Geological Survey, which is in the public domain in the United States; esker by Phil Camill, published under a Creative Commons Attribution-NonCommercial-NoDerivs 2.0 Generic (CC BY-NC-ND 2.0) license.