In the Emerson poll that came out last night they had breakdowns for each of Iowa's four congressional districts. Huge margins of error since they are calculated from smaller subsets of the same dataset used for the statewide poll, but it looks like:
IA1: Biden, Sanders, and Klobuchar should easily hit 15% but the others (including Warren, Yang, and Buttigieg) are below the threshold district wide and may be nonviable in a lot of precincts.
IA2: Only Biden and Sanders clearly above 15%. Warren and Buttigieg are close, everyone else super low.
IA3: Only Biden and Sanders clearly above 15%. Klobuchar is close.
IA4: Only Biden and Sanders clearly above 15%, Yang, Warren and Buttigieg are close.
If a candidate gets above 15% of state delegate equivalents in any congressional district (after the viability threshold and realignment in individual precincts), they'll receive at least one of Iowa's delegates to the national convention (and an automatic entrance to the next debate, although everyone listed has already qualified based on polls).
Since Sanders and Biden are the current frontrunners and have dramatically different policies and perceived strengths/weaknesses, my guess is most people caucusing for other candidates are doing so because they really don't like either of the top two options. Could see some odd marriages of convenience on caucus night. Klobuchar supporters switching over to Buttigieg to make him viable or vice versa?