Is there a meassure word missing in 在这河里游泳危险
If you break down &quot;在这河里游泳危险&quot; you can see a verb is missing.

&quot;Swimming in this river (is) dangerous&quot;
It should be either &quot;在这河里游泳(有)危险&quot; or &quot;在这河里游泳(很)危险&quot;

The classifier 条 is optional. when we are not counting the number of river. 这 (this) already indicated it is only one particular river

&quot;这河/ 这条河&quot;  = &quot;this river&quot; - You can omit the classifier 条 because there is no counting word

&quot;这一条河&quot; = &quot;this one river&quot; - You cannot omit the classifier 条 and write &quot;这一河&quot; because there is a counting word 一 here

&quot;这三条河&quot; = &quot;these three rivers&quot; - You cannot omit the classifier 条 and write &quot;这三河&quot; because there is a counting word 三 here