Where was Maggie when Mrs Tulliver looked out of the window? Why was Mrs. Tulliver concerned?

From the text:

An’ now you put me i’ mind,” continued Mrs Tulliver, rising and going to the window, “I don’t know where she is now, an’ it’s pretty nigh tea-time. Ah, I thought so,—wanderin’ up an’ down by the water, like a wild thing: She’ll tumble in some day.”