uruḥ kakṣo na ghāṅghyaḥ ||
The correct verse is probably this:
RV_6,045.31a adhi bṛbuḥ paṇīnāṃ varṣiṣṭhe mūrdhann asthāt |
RV_6,045.31c uruḥ kakṣo na gāṅgyaḥ ||
Based on the context, wide (uru) and girth or bank (kakṣa), etc. the verse appears to be referring to the river.
R. L. Kashyap translates it the same way:
6.45.31: Bṛbu presided over the powerful places of the Paṇis, like the wide bank of the Ganges.
[ varṣhiṣhthe: most powerful, (5.7.1); According to Sāyaṇa, Bṛbu is a divine artisan (takṣha). The three mantras 31-33 deal with his gifts. Bṛbu obtained his possessions from the Paṇis for doing their work. Recall that Paṇis are the traffickers in the powers of senses. ]