I want to know that was the river Brahmaputra created by Lord Brahma or any other God?
Chapter 81, (Kāmarūpamahātmyavarṇana) of Kālikāpurāna describes the origin of the river Brahmaputra in the following way.
I'm providing the Summarized English Translation of the translated Hindi text
Forermerly in the vicinity of the Nilāchala hills were situated many kundas, like the Apunarbhava kunda (named such because the person who bathed here would never be reborn again), bathing in which people were obtaining permanent residence in svarga directly after death by the grace of it's presiding deity, Bhagavati Kāmakhyā. Indra complained to Brahmā that svarga was getting overpopulated with people bathing in those kundas & Yama complained that sinners were evading justice by bathing there. So Brahmā made humanity forget about the tirtha-mahatmya of the Yonipitha & proceeded there to conceal the sacred kundas of the site.
On the way, he was hit by the arrows of Kāmadeva on seeing a ṛṣipatnī named Amoghā & deposited his seed on that spot. That seed was implanted in Amoghā's womb by her husband, a sage named Śāntanu, which led to the birth of Lauhitya. Lauhitya's physical appearance was that of a boy of crimson-red complexion wearing blue-garmets & gem-studded garland, holding a book & a lotus in his right arms, a spear & flag in his left arms, seeated on a dolphin & surrounded by a mass of clear water, so Śāntanu had him placed inside a artificially constructed kunda (identified as Parashurama kunda of south-eastern Arunachal Pradesh from where the river Lohit originates) in the mountains. This Lauhitya-kunda was visited by Paraśurāma on his quest to relieve himself from the sin of matricide. Paraśurāma, on being relieved from matricide after bathing in it, decided to make the waters of Lauhitya-kunda available to the massess. So he carved out a channel out of the Lauhitya-kunda into the sea which inadvertedly passed through Kamarupa-kshetra & submerged the kundas of Yonipitha within its waters. The channel dug by Paraśurāma became the river Lauhitya aka Brahmaputra.
On a sidenote, a legend is prevalent among Hindus of eastern Bengal that while approaching near the sea, Lauhitya was distracted by the beauty of a local rivulet named Śitalakhyā & deviated from the channel carved by Paraśurāma to merge with her, which caused Paraśurāma to stop dragging his plough (with which he had been carving out Brahmaputra's channel till then) & cursed Lauhitya to lose his sin-relieving properties. However on Lauhitya's request, he decreed that Lauhitya would possess his sin-relieving powers only on the 8th day of the bright fortnight in the month of Caitra. On that day, Hindus of eastern Bengal gather in large numbers at the banks of the old riverbed of Brahmaputra at Laṅgalabandha (where Parashurama is believed to have pronounced the curse on Lauhitya) to take bath in the river uttering the following pranāma-mantra
ब्रह्मपुत्र महाभाग शान्तनोः कुलनन्दन । अमोघागर्भसम्भूत पापं लौहित्य मे हर ।। त्वं ब्रह्मपुत्र भुवनतारण तीर्थराज गम्भीरनीर परिपूरित सर्वदेह । त्वद्दर्शन हरतु मे भवघोरदुःखं संयोगतः कलियुगे भगवन्नमस्ते ।।
No, the Brahmaputra river was not created by Brahma.
It was actually created by Parashuram.
Parashuram, after beheading his mother, found his axe sticking to his palm because he had committed a sin.
To relive the sin, he went to several places, but without any success.
Finally, he went to Manasarovar, where he could relieve himself of the sin.
He then decided to make way for the water in the sarovar to go to various places on the earth as a river. The river is Brahmaputra.