A word ending in a case-affix is compounded with words denoting the names of rivers, when the compound word denotes a thing other than that expressed by the terms of the compound and is an appellative, the compound so formed being an avyayībhāva ,
The anuvritti of the word samkhyå does not extend to this sutra. Though this rule is given in the sub-division relating to optional compounds it is, however, a Nitya-samasa rule : for no Name (सञ्ज्ञा) can ever be expressed by a sentence, and that being so, these compounds can never be analysed. As उन्मत्तगङ्गम् ' the country called Unmatta Ganges .' So also लोहितगङ्गम्, कृष्णगङ्गम् ; शनैर्गङ्गम्.
Why do we say 'when it denotes a thing other than that expressed by the component parts of the compound' ? Observe कृष्णवेण the 'river Krishnavena.'
Why do we say 'when it expresses an Appellative' ? Observe शीर्घो गङ्गो देशः ' the country where the Ganges moves rapidly.',