1908 Olive Bray in her Edda Saemundar as “The Sayings of Grimnir”: 

"Thunder-flood. The river name Thund may thus be connected with Icelandic þunor by the suffix þ (V), or, meaning Swollen, with Icelandic. þindan (B). The 'fish of the mighty Wolf' is according to G. the sun, or prey of the wolf of darkness, st. 39 : she shines in the heavens till swallowed by Fenrir ; see Vafthrudnismal 46. cf. Dieter & Heinzel who translate the Wolf's flood or stream which flowed from his jaws, and connect the passage with the storming of Asgarth by the Wanes mentioned in Voluspa 24. 21. See Voluspa st. 43." 

1923 Henry Bellows in his The Poetic Edda as “Grimnismol: The Ballad of Grimnir”:

"Thund — 'The Swollen' or 'Roaring', river surrounding Valhall. Thjothvitnir's fish, Sol, pursued by the wolf Skoll. Thjothvitnir - 'Mighty Wolf'. "

1967 W. H. Auden & P. B Taylor in The Elder Edda as “The Lay of Grimnir”:
 
"The fish of the wolf is the sun, caught and devoured at Ragnarök by the wolf, Sköll (cf. 'Song of the Sybil')."

1984 Rudolf Simek, A Dictionary of Northern Mythology:

"Þund (ON 'the roaring one'?). A river in front of Valhall, according to Grímnismál 21."

"Þundr (or Þudr? ON). A name given to Odin in Grímnismál (21, 46, 54) and the Hávamál 145. The meaning of the name is obscure, but is probably related to OE ðunian 'swell'."

1996 Carolyne Larrington in The Poetic Edda as “Grimnir’s Sayings”:

"Thund: This verse is very obscure. Thund is probably a river in which the 'wolf's fish', possibly the World-serpent swims. Those 'rejoicing in slaughter' must be the Einherjar on their way to Valhall."

2011 Andy Orchard in The Elder Edda as "The Lay of Grimnir":

"A very obscure stanza: Thund elsewhere (Hávamál 145, Grímnismál 46, 54) is a name of Odin, but it might be a river; 'Mighty Wolf's fish' may refer to the Midgard-serpent if 'Mighty Wolf' (Þjóðvitnir) is a name of Fenrir (cf. 'Famed Wolf' [Hróðvitnir] in 39 and Lokasenna 39.)"

The following is a slightly modified excerpt from  When is a fish a bridge? An Investigation of Grímnismál 21 by Eysteinn Björnsson. Please refer to his site for the complete and original form of the article:
 
Grímnismál has been said to be the most obscure of the Eddaic poems. It is first and foremost a "cosmological" poem, but one couched in densely allegorical language, the interpretation of which has stumped many scholars. This stanza is perhaps the most obscure of all, and has never been satisfactorily explained:

Excerpts from Viktor Rydberg's Investigations into Germanic Mythology, Vol. I, no 93:
 
"Here (in Grímnismál 21), as in Fáfnismál 15, the air is compared with a river, in which the horses are compelled to wade or swim if the bridge leading to Asgard is not used, and the current in this roaring stream is said to be very strong; while, on the other hand, "the fish" stands safe therein. That the author of Grímnismál called the bridge a fish must seem strange, but has its natural explanation in Icelandic usage, which called a bridge-end or bridge-head a sporður, that is, a fish-tail. Compare Sigurdrífumál 16, which informs us that runes were risted on "the fish-tail" of the great mythic bridge (á brúar sporði), and the expression brúarsporður (bridge-head, bridge-"fish-tail") in Njáls Saga ch. 145 and Biskupasögur 1, 17. As a bridge-pier could be called a fish-tail, it was perfectly logical for the poem to make the bridge a fish. On the zenith of the bridge stands Valhall, that secures those fallen in battle.
 
"It is Bifrost's northern bridge-head which particularly requires the vigilance of Heimdall, the ward of the gods, since the frost-giants and the damned of Niflhel are its neighbors. Heimdall is therefore "widely known" among the inhabitants of Niflhel (Skírnismál 28), and Loki reproaches Heimdall that his vocation as watchman compels him to always expose his back to the torrents of an unfavorable sky (Lokasenna 48). In the night which constantly broods over this northern zone shine the forms of the "white" god and of his gold-beaming horse Gulltoppur. His eye penetrates the darkness of a hundred rasts, and his ear catches the faintest sound (Gylfaginning 27). Near Bifrost, presumably at the bridge-head itself, the mythology has given him a fortified citadel, Himinbjörg, "the fortress of heaven" with a comfortable hall well supplied with "the good mead" (Grímnismál 13; Gylfaginning 27).
 
"The lower world is more extensive in all directions than the surface of the earth above it. Bifrost passes outside and below the crust of the earth to rest with its bridge-heads on the foundation of the lower world, upon which everything else is built. The lower world is therefore called Jörmungrund, "the great ground or foundation" (Hrafnagaldur Óðins 25), and its uttermost zone, jaðarr Jörmungrundar, "the domain of the great ground," is open to the celestial canopy. The under side of the earth is not its roof.
 
"From Hliðskjálf, the outlook of the gods in Asgard (Hrafnagaldur Óðins, the prose introduction to Skírnismál and in Grímnismál), the view from Odin’s throne extends to Midgard, to the sea, and to the giant-world situated beyond the Elivogar rivers (see the texts mentioned), and accordingly should also extend to the broad zone of Jormungrund, excepting its northernmost part, which is always shrouded in night and fog (nifl = mist). From Hliðskjálf Odin’s eye cannot discern what is done there. It is Heimdall who keeps watch there, and when anything unusual is perceived Odin sends the raven Huginn (Hugur) there to spy it out (Hrafnagaldur Óðins 10, 3). But from Hliðskjálf, the earth conceals all that part of Jormungrund below it; and as it is important to Odin that he should know all that happens there, Hugin and Munin fly daily over these subterranean regions: Huginn og Muninn fljúga hverjan dag jörmungrund yfir (Grímnismál 20). The expeditions of the ravens over Niflhel in the north and over Surt's "deep dales" in the south expose them to dangers: Odin expresses his fear that some misfortune may befall them on these excursions (Grímnismál 20).
 
"The modern conception of the removal of those fallen by the sword to Asgard is that the valkyries immediately carry them through blue space to the halls above. The heathens did not conceive the matter in this manner.
 
[Editor’s interlude: In Fáfnismál 10, all men are said to go to Hel, and in Gisli Sursson’s Saga ch. 24, ‘hel-shoes’ are bound on a warrior destined for Valhalla; in several other passages, warriors who die in battle are said to be “beaten to Hel”, “sent to Hel” and the like. These are strong indicators that according to the heathen conception even warriors destined for Valhalla first arrived in Hel, before preceding there. In heathen graves, warriors and chieftains are often interred with horses, wagons, ships and other means of transport. In the mythology, all of these methods are used to travel to the underworld. Rydberg locates Urd’s well there. The gods are said to sit in judgment there daily, and in Solarljod 51, dead men sit “in Norn’s seats” for nine days before going to their ultimate fate. The logical conclusion is that the gods judge the dead by Urd’s well daily. The goddess of fate is the goddess of death (but not the same as Loki’s daughter, whose does not appear in Eddic poems). After naming a series of rivers which “fall to Hel”, Grimnismal 29-30 places Asgard, the home of the gods, on one end of Bifrost, and Urd’s well on the other. Asgard is located in the heavens. The well is located under a root of Yggdrassil. In northern Europe, the roots of trees are located below the earth without exception. Setting aside Snorri’s placement of Asgard (which he identifies with the city of Troy in Asia) on earth and Urd’s well in the heavens, the most natural reading of these verses indicates that Bifrost extends from heaven to Hel without ever touching the earth. The image of Bifrost as the rainbow confirms this. In association with this, several sources speak of a road to Hel, upon which all dead men must travel.

In the poem Eiríksmál (Fagrskinna, ch. 8), Odin welcomes a king and his retinue into Valhal. The imagery makes clear that these warriors arrive there by riding across the Bifröst bridge:

"The bridge Bifrost would not have been built or established for the daily connection between Asgard and Urd's subterranean realm (Grímnismál 28-29) if it had been unnecessary in the mythological world of fancy. Mani's way in space would not have been regarded as a road in the concrete sense, that quakes and rattles when Thor's thunder-chariot passes over it (mána vegur dundi - Haustlöng 1, Skáldskaparmál 25), had it not been thought that Mani was safer on a firm road than without one of that sort. To every child that grew up in the homes of our heathen fathers the question must have lain near at hand, what such roads and bridges were for, if the gods gained no advantage from them. The mythology had to be prepared for such questions, and in this, as in other cases, it had answers with which to satisfy that claim on causality and consistency which even the most naive view of the world presents. The answer was:
 
 "If the Bifrost bridge breaks under its riders, as will happen in course of time, then their horses would have to swim in the sea of air (Bilröst brotnar, er þeir á brú fara, og svima i módu marir — Fáfnismál 15; A horse does not swim as fast and easily as it runs. The different possibilities of travel are associated with different kinds of exertion and swiftness. One method is more practical than the other. The solid connections which were used by the gods and which the mythology built in space are, accordingly, useful and convenient. The valkyries, riding at the head of their chosen heroes, as well as the gods, have found solid roads advantageous, and the course they took with their favorites was not the one presented in our mythological textbooks. Grímnismál str. 21 informs us that the breadth of the atmospheric sea is too great and its currents too strong for those riding on their horses from the battlefield to wade across (árglaumur þykir ofmikill valglaumi að vaða)."