The affix kap is added to a bahuvrīhi samāsa the last member of which is a feminine word ending in long ī or ū defined as nadī words or which ends in a short ṛ,
The word नदी is defined in 1.4.3. Thus बहुकुमारीको देशः (बह्व्यः कुमर्योऽस्मिन्देशे), बहुब्रह्मबन्धूकः ॥ बहुकर्तृकः ॥ The त in ऋत् is for the sake of euphony.,