you're right, I misunderstood the article. nikk does have the 
"pissing and shitting all over the place" definition but daij 
don't, so I guess it's fine as it is.

without reading the article, i don't think they're suggesting that the pollution is coming from people pissing and shitting all over the place (i.e., the suggested nikkoku sense); they're suggesting that the existing toilets are overflowing and discharging waste all over the mountain, creating "white rivers".  it clearly seems like the "discharge" sense to me

@@ -21,1 +21,2 @@
-<gloss>soiling one's pants</gloss>
+<gloss>soiling oneself</gloss>
+<gloss>wetting one's pants</gloss>
@@ -27,0 +28,1 @@
+<gloss>effluence</gloss>

”大小便を所かまわずたれ流すこと。また、直腸や尿道などの括約筋（かつやくきん）
がゆるんで、大小便を無意識にたれること。" nikk
which fits well together with the usage in this yomiuri 
article: "富士山の５合目以上には山小屋なども含め計約５０か所のトイレがある
が、かつては汚物の垂れ流しでトイレットペーパーが山肌にへばりつく「白い川」が問
題となっていた。"
http://www.yomiuri.co.jp/eco/news/20130825-OYT1T00001.htm
in a way that "discharge" or "incontinence" does not.
_
though prog does have
"II 〔出しっぱなしにすること〕
		し尿を川に垂れ流しする
let raw sewage flow into a river
		有毒廃水の垂れ流しをする
discharge toxic waste water untreated"

should there be a new sense for "大小便を所かまわずたれ流すこと"?

Commonly used.

@@ -8,0 +8,3 @@
+</k_ele>
+<k_ele>
+<keb>たれ流し</keb>