Manava Dharmashastra 5.107

5.107

nadyaḥ pārāvāre codyodakāyā aśuddhathupahate | tasyā eva vegaṃ gatāyāḥ kūlaṃkaṣāyā udakaṃ vegena śudhyati | na yathānyasyā bhūmeḥ "bhūmiḥśudhyati pañcabhiriti” ( 123 ) śuddhaye pratītinaivaṃ nadītīreṣu | athavā vegavatyā aśucipravāhasaṃsargeṇāśucyāśaṅkāyāmidamucyate nadī vegeneti | naivaṃ mantavyaṃ itaścāmutaścāśucipravāhaiḥ saṃsṛṣṭā na śudhyati | śārīre vyabhicāre'nupalabhyamāne parapuruṣarūpaguṇānucintanena manoduṣṭā rajasā ṛtau śoṇitena srutena śudhyati strī | saṃnyāsaḥ ṣaṣṭhe vakṣyate | tena dvijottamāḥ śuddhā bhavanti | na kathañcinmānasāpacāre | yadviduṣā cintitaṃ sūkṣmaprāṇivadhādi tadeṣāmapanīyate |

Medhātithi’s commentary (manubhāṣya):
When the banks of a river with water shallowed down becomes defiled by unclean things, its water becomes purified by the current of the same river, when it has regained its current strong enough to demolish its sides. The brinks of rivers are not purified in the manner in which other ground is purified “by means of five things” (Verse 123).

Or, it may be that the text has declared that ‘the river is purified by its current’ in view of the idea that people may have in regard to the river having become defiled on account of un clean things flowing along its current; and the meaning is that it should not be thought that, inasmuch as the river has become contaminated by the flowing along of unclean things coming from all sides, it can never, become pure.

The woman who has not been found to have had carnal intercourse with any man, but continues to think of the beauty and good qualities of other men, is regarded as ‘of undean mind’, and such a woman becomes purified by ‘menstruation’; i.e.,; by the flow of blood during her courses.

‘Renunciation’ shall be described in Discourse VI, and by this are Brāhmaṇas purified. And no mere mental process removes the sin that they, in their ignorance, may have committed in the shape of having entertained thoughts for the killing of small insects and so forth.—(107).