The journal thus describes the stream:-- "Its width at the entrance is one hundred and fifty yards; on going three miles up, Captain Lewis found it to be of the same breadth and sometimes more; it is deep, gentle, and has a large quantity of water; its bed is principally of mud; the banks are abrupt, about twelve feet in height, and formed of a dark, rich loam and blue clay; the low grounds near it are wide and fertile, and possess a considerable proportion of cottonwood and willow.

It seems to be navigable for boats and canoes; by this circumstance, joined to its course and quantity of water, which indicates that it passes through a large extent of country, we are led to presume that it may approach the Saskaskawan (Saskatchewan) and afford a communication with that river.

The water has a peculiar whiteness, such as might be produced by a tablespoonful of milk in a dish of tea, and this circumstance induced us to call it Milk River." Modern geography shows that the surmise of Captain Lewis was correct.
Some of the tributaries of Milk River (the Indian name of which signifies "The River that Scolds at all Others") have their rise near St.Mary's River, which is one of the tributaries of the Saskatchewan, in British America.
The explorers were surprised to find the bed of a dry river, as deep and as wide as the Missouri itself, about fifteen miles above Milk River.
Although it had every appearance of a water-course, it did not discharge a drop of water.

Their journal says:-- "It passes through a wide valley without timber; the surrounding country consists of waving low hills, interspersed with some handsome level plains; the banks are abrupt, and consist of a black or yellow clay, or of a rich sandy loam; though they do not rise more than six or eight feet above the bed, they exhibit no appearance of being overflowed; the bed is entirely composed of a light brown sand, the particles of which, like those of the Missouri, are extremely fine.

Like the dry rivers we passed before, this seemed to have discharged its waters recently, but the watermark indicated that its greatest depth had not been more than two feet.