腹 (fuku, "belly") is used here in an old, specific sense meaning "thick" or "deep," rather than its everyday meaning — 水沢腹堅 describes ice growing genuinely thick across the surface of mountain streams and marshy ground (沢, sawa), rather than the thin skin that had already been forming on standing water since early winter. This kō's window is statistically Japan's coldest of the entire year — the point most sources place the annual low temperature, right in the middle of Daikan.
The ice doesn't thicken in one smooth process; it repeatedly partially melts under a bit of midday sun and refreezes overnight, each cycle adding a further layer — a slow, uneven accumulation rather than a single hardening. What looks from a distance like unmoving solid ice is, up close, the record of many small freeze-thaw cycles stacked on top of each other.
At peak season, especially prized for its shirako right now.
In excellent season, plump and full-flavored through the coldest weeks.
Continuing in good season, its natural sweetness a welcome contrast to the cold.
Still at its best, following straight on from the previous kō.
Still sweet and central to nabe cooking through the season's coldest stretch.
Suisen rejoins here, continuing from earlier in winter — narcissus blooms across an unusually long window of the traditional calendar, one of the few flowers genuinely present through nearly the whole of Daikan and the sekki before it.
Water drawn during this window, the very coldest of the year, was traditionally believed the purest and most resistant to spoiling of the whole calendar — the same underlying belief behind kanjikomi brewing a few weeks earlier at Shōkan, but sharpened here at the actual seasonal low. It was preferentially used for the final stages of miso and soy sauce fermentation, for pounding mochi, and, in some regions, kept stored through the year as a folk remedy believed to ward off stomach ailments.
"If winter has come, can spring be far behind?" — A saying most familiar in Japan via Ueda Bin's early-1900s translation of Shelley's "Ode to the West Wind," which entered Japanese proverbial use almost as if it were native — a rare case of a Western Romantic line becoming a genuinely Japanese seasonal saying, fittingly paired with the calendar's actual coldest week.
腹 (fuku) ordinarily means "belly" or "stomach," but in this kō's title carries an older, secondary sense of "thick" or "deep" — one of several places in the seventy-two kō where a character is doing work well outside its everyday modern meaning, a small linguistic fossil from the calendar's older Chinese source text.
The visible bands and cloudy sections are usually a record of separate freeze-thaw cycles, not a single uniform freeze.
This is traditionally considered the single best window of the entire year for kan no mizu, if you're inclined to try the custom.
"At Naniwa Bay, the channel markers stand locked in ice — I see in this a sign of just how deep winter has become."
Statistically the coldest stretch of the year for most of Japan — a sensible week to double-check winterizing rather than assume it's already handled.
Risshun, the traditional first day of spring, falls only about ten days past the end of this kō's window.
Teika reads the ice not as an obstacle but as a measuring instrument — the deeper the freeze, the more precisely it tells him where in the season he stands. It's a strange, useful reframe: the hardest conditions aren't only something to get through, they're also the clearest evidence of exactly how far through you already are. What in your own life, however uncomfortable right now, might actually be telling you precisely how much progress you've made rather than how much is still ahead?