网友的这个问题从以下几个方面来分析一下：

1. 介词 in 和 on 与 river 类词搭配：

一般说来 in the river 表示“在河里”。如：

We watched the fish swimming in the river. 我们看着鱼儿在河里游。

They rowed along a loop in the river. 他们在河里绕圈划行。

The children splashed about in the river all afternoon. 孩子们整个下午都在河里嬉水。

The three bodies found in the river remain unidentified. 河里发现的三具尸体仍然身份不明。

用 on the river 表示“在河边”“在河畔”。如：

Cairo is on the river Nile. 开罗位于尼罗河畔。

London is on the Thames. 伦敦坐落在泰晤士河畔。

They've built a new house on the riverside. 他们在河畔建了一座新房子。

She lives in a drowsy little village on the river. 她住在河边一个宁静的小村子里。

He visited relatives at their summer house on the river. 他拜访了住在河畔避暑别墅的亲戚。

若表示“在河面上”通常用介词 on。如：

There was a rowing boat on the river. 河上有一只划艇。

They live in a houseboat on the river. 他们住在河上的一间船屋。

Boat traffic on the river is heavy in summer. 河上的船只流量在夏季非常大。

A flock of Canada geese landed on the river in front of us. 一群加拿大鹅落在我们前面的河面上。

The ice on the river is thick enough to skate on / across / over. 河上的冰够厚，可以在上面滑冰。

2. Southbank的理解

Southbank（也可写成 South Bank）意思是“泰晤士河南岸”（与泰晤士河南岸相连的地区，尤指位于伦敦威斯敏斯特与黑修士桥之间的文化中心），英语词典的解释是： the area adjacent to the southern bank of the River Thames, especially the cultural complex located between Westminster and Blackfriars Bridges in London.

3. 关于河中的弯道

根据常识，河中的弯道（bend）肯定是在“河里面”，不可能是在“河面”或“河畔”。如：

The line of trees doubles back on itself at a bend in the river. 那排树在河道拐弯处折回来。

The bend in the river had cause an eddy of fast swirling water. 河流转弯处形成一个快速旋转的漩涡。

Soon you come to a point where there’s a bend in the river. 很快你们就会来到河流的一个转弯处。

No one knew what lay in store for us round the next bend in the river. 谁都不知道在下一个河湾将有什么在等待着我们。

4. 回到高考题中的这个句子

Southbank, at an eastern bend in the Thames, is the center of British skateboarding, where the continuous crashing of skateboards left your head ringing.

句中 at an eastern bend in the Thames 应该是修饰 Southbank 的非限制性定语，at an eastern bend in the Thames 的意思是“泰晤士河向东拐弯的地方”。上面说过，Southbank 虽然内含 bank，而且翻译成汉语也是“南岸”，但它其实不是仅仅是“岸”，而是一个地方，一个区域（area），即 the area adjacent to the southern bank of the River Thames, especially the cultural complex located between Westminster and Blackfriars Bridges in London。

句子大意：Southbank, at an eastern bend in the Thames, is the center of British skateboarding, where the continuous crashing of skateboards left your head ringing. 位于泰晤士河向东拐弯处的南岸是英国滑板运动的中心，滑板不断的撞击声让你头晕目眩。