As an extension to, rather than a disagreement with, Gareth Rees answer, it is an extended metaphor of money as a liquid substance. But beyond that, it's using an existing extended metaphor in finance, economics, and everyday English. The change from liquid to solid - freezing - is one type of phase change. And in Capital Vol 1, that's how Marx describes the conversion of material commodities into money.
Now let us examine the circuit M—C—M [Money-Commodity-Money] a little closer. It consists [...] of two antithetical phases. In the first phase, M—C, or the purchase, the money is changed into a commodity. In the second phase, C—M, or the sale, the commodity is changed back again into money. The combination of these two phases constitutes the single movement whereby money is exchanged for a commodity, and the same commodity is again exchanged for money; whereby a commodity is bought in order to be sold, or, neglecting the distinction in form between buying and selling, whereby a commodity is bought with money, and then money is bought with a commodity.
By the twentieth century, economists are using "liquidity" to describe why cash money is more useful than assets which are more difficult to sell. Famously, John Maynard Keynes in The General Theory:
But this decision having been made, there is a further decision which awaits him, namely, in what form he will hold the command over future consumption which he has reserved, whether out of his current income or from previous savings. Does he want to hold it in the form of immediate, liquid command (i.e. in money or its equivalent)? Or is he prepared to part with immediate command for a specified or indefinite period, leaving it to future market conditions to determine on what terms he can, if necessary, convert deferred command over specific goods into immediate command over goods in general? In other words, what is the degree of his liquidity-preference — where an individual’s liquidity-preference is given by a schedule of the amounts of his resources, valued in terms of money or of wage-units, which he will wish to retain in the form of money in different sets of circumstances?
And this sense of liquidity is the finance jargon we see used in the market today, to refer to something that is easy to sell, or the volume of freely trading items on a commodity market. Atwood has taken a submerged metaphor that has worked its way from scholarship into everyday English, and followed through with its poetic implications: money flowing through the room like river water, freezing into fixed capital, and being transformed into opulent material.