Did the chandra gupta gain control of the commerce on the ganges river by marriage

Anda dynasty had full control over movement & trade on the River Ganga. Chandragupta Maurya after eliminating the Nanda brothers (sparing the old Emperor Mahapadma Nanda, who was his biological father) and assuming the title of Emperor, had automatically came into power over that trade.  

The only significant struggle in this direction was to win over Rakshasa Mantri, the Prime Minister of Nandas. Chandragupta's mentor & King-maker, Chanakya advised his protege to get (his rival) Rakshasa on his side, as he was capable of adminstrating the Empire well. Rakshasa was so loyal to his erstwhile masters (though arrogant & incompetent) that he plotted to kill Chandragupta Maurya, using his nubile daughter as a ploy or decoy. When all efforts failed, Chandragupta appealed to his good sense, inevitability & convinced him to take over the responsibility of the Imperial administration. The trade & that too on the waterway of River Ganga was part of this package. With administration in firm hands, the Emperor was free to plan out his military campaigns as in the interregnum the regional Satraps were entertaining thoughts of breaking free from the Empire.

Nanda dynasty had full command over transportation & trade on the River Ganga. Chandragupta Maurya after killing the Nanda brothers (forgiving the old Emperor Mahapadma Nanda, who was his biological father) and considering the privilege of Emperor, had automatically got into control over that business.

The exclusive important effort in this way was to gain over Rakshasa Mantri, the Prime Minister of Nandas. Chandragupta's instructor & King-maker, Chanakya encouraged his protege to take (his opponent) Rakshasa on his team, as he was proficient at administrating the Empire properly. Rakshasa was so faithful to his past mentors (though proud & inefficient) that he planned to eliminate Chandragupta Maurya, utilizing his nubile child as a ploy or imitation. When all attempts deserted, Chandragupta requested to his real soul, inevitability & persuaded him to take over the obligation of the Imperial government. The trade & that too on the waterway of River Ganga was a section of this case. With the government in strong hands, the Emperor was free to draw out his army operations as in the suspension the territorial Satraps were interesting ideas of cutting loose from the Empire.

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nothing certain is known about what happened, but it seems that the country declined gradually, perhaps due to bad government, or, more likely, due to a series of bad harvests, which resulted in widespread and recurrent famine.