If her ex is the only source of information saying that she and the boy went to that river, then I am unsure if they ever arrived there.
Anyone could have driven her vehicle there and laid out the blanket.
This stands out for me.
"Jansen and Walker were partners four years ago, he said, and since then Jansen said
Walker has tried to move away with the seven-year-old three times, including twice to Ottawa and once to the Victoria area."
link
What that says to me is that the married father of a young girl, who parted ways with Dawn four years ago, shares custody of a seven year old son with Dawn. Because he is an active father, Dawn is prevented from taking the boy to another city. That is, the boy is entitled to have a relationship with his father and therefore the child must remain in Saskatoon as long as the father lives there as well.
I could see Dawn viewing that as controlling or worse, but Canadian law looks at the best interests of the child and does not allow one parent to deprive the child of regular contact with the other parent. Allegations of domestic violence would not be enough to change the rights of the child because courts do not view spousal conflict as indicative of violence towards children.
"Earlier in the day, he took his son to a day camp at the Shaw Centre on Saskatoon’s west side. Jansen said he picked Vincent up afterward, and then went with his wife and their daughter to get pizza, and then to Kinsmen Park.
“We just had a really nice time and we rode the little train they have there,” Jansen said. “Vincent really wanted to sit in the caboose of that kids train there. I
dropped him off with his mom … at about six o’clock, and that’s the last time I saw him.” (same link)
This suggests that the 7 year old boy had a really nice day with his father, sister and step-mother before joining his mother at 6 PM. After a long day at day-camp, dinner, visiting the park and riding the mini-train in 22°C
weather (71°F), his mother took him to Chief Whitecap Park around 6:30 PM. That's a long day for a 7 year old.
Hopefully there's dashcam footage of Dawn driving to the river to help focus the search.