Interlake Invasion: It was La Vérendrye's sons, Jean-Baptiste and Pierre, who were the first to reach Lake Winnipeg in the summer of 1733. Following the Winnipeg River northwest to its mouth at the Lake, the young men explored the shoreline of the vast lake. In 1734, they constructed a fort on the Red River, about 5 miles north of the present-day city of Selkirk. Between 1741 and 1743, young Pierre pushed far to the north, into the Interlake district of Manitoba, founding forts on Lake Winnipegosis, Cedar Lake, the northern tip of Lake Winnipeg, and the Saskatchewan River.

By the 1740's, La Vérendrye's trade network had fulfilled its original, "unofficial" purpose. The Hudson's Bay monopoly had been broken, and furs were now flowing steadily to Montreal. As early as 1732, when only 2 of La Vérendrye's posts had been established, trade at the HBC's York Factory post had dropped by 15%. The renegade tactics of the French fur traders forced the stolid Hudson's Bay Company to revise its stay-at-home corporate policy.

But as far as La Vérendrye's "official" sponsors were concerned, the family's preoccupation with empire building had undermined the search for the Western Sea. Constantly threatened with the withdrawl of the King's support, La Vérendrye resorted to renting his fur trading posts in order to keep his creditors at bay. His longtime business ally, Governor Beauharnois, was often forced to intercede on his behalf.

Throughout many setbacks and personal tragedies, including the loss of his eldest son, Jean-Baptiste in an attack by the Sioux, La Vérendrye persevered. In 1749 he was awarded the cross of Sainte-Louis, in honour of his career, and was given permission to carry on further exploration west of the Saskatchewan River. Just as he was making plans to leave for yet another far-flung expedition in December of 1759, Pierre Gaultier de la Varennes de la Vérendrye passed away.